perf: add perfmoonwalk command to walk the changelog backwards

This lets us test the effectiveness (or ineffectiveness) of the revlog chunk
cache when walking revlogs backwards.
revlog: read/cache chunks in fixed windows of 64 KB

When reading a revlog chunk, instead of reading up to 64 KB ahead of the
request offset and caching that, this change caches a fixed window before
and after the requested data that falls on 64 KB boundaries. This increases
cache hits when reading revlogs backwards.

Running perfmoonwalk on the Mercurial repo (with almost 20,000 changesets) on
Mac OS X with an SSD, before this change:

$ hg perfmoonwalk
! wall 2.307994 comb 2.310000 user 2.120000 sys 0.190000 (best of 5)

(Each run has 10,668 cache hits and 9,304 misses.)

After this change:

$ hg perfmoonwalk
! wall 1.814117 comb 1.810000 user 1.810000 sys 0.000000 (best of 6)

(19,931 cache hits, 62 misses.)

On a busy NFS share, before this change:

$ hg perfmoonwalk
! wall 17.000034 comb 4.100000 user 3.270000 sys 0.830000 (best of 3)

After:

$ hg perfmoonwalk
! wall 1.746115 comb 1.670000 user 1.660000 sys 0.010000 (best of 5)
revlog: allow tuning of the chunk cache size (via format.chunkcachesize)

Running perfmoonwalk on the Mercurial repo (with almost 20,000 changesets) on
Mac OS X with an SSD, before this change:

$ hg --config format.chunkcachesize=1024 perfmoonwalk
! wall 2.022021 comb 2.030000 user 1.970000 sys 0.060000 (best of 5)

(16,154 cache hits, 3,840 misses.)

$ hg --config format.chunkcachesize=4096 perfmoonwalk
! wall 1.901006 comb 1.900000 user 1.880000 sys 0.020000 (best of 6)

(19,003 hits, 991 misses.)

$ hg --config format.chunkcachesize=16384 perfmoonwalk
! wall 1.802775 comb 1.800000 user 1.800000 sys 0.000000 (best of 6)

(19,746 hits, 248 misses.)

$ hg --config format.chunkcachesize=32768 perfmoonwalk
! wall 1.818545 comb 1.810000 user 1.810000 sys 0.000000 (best of 6)

(19,870 hits, 124 misses.)

$ hg --config format.chunkcachesize=65536 perfmoonwalk
! wall 1.801350 comb 1.810000 user 1.800000 sys 0.010000 (best of 6)

(19,932 hits, 62 misses.)

$ hg --config format.chunkcachesize=131072 perfmoonwalk
! wall 1.805879 comb 1.820000 user 1.810000 sys 0.010000 (best of 6)

(19,963 hits, 31 misses.)

We may want to change the default size in the future based on testing and
user feedback.
branchmap: add documentation on the branchcache on-disk format
branches: avoid unnecessary changectx.branch() calls

This requires reading from the changelog, which can be costly over NFS.

Note that this does not totally remove reading from the changelog; we
still do that when calling changectx.closesbranch(). That call will be
removed in a later patch.

Running hg branches on the PyPy repo (with 996) over a busy NFS server,
before this change:

$ time hg --profile branches > /dev/null
   CallCount    Recursive     Total(s)    Inline(s) module:lineno(function)
        2042            0      2.2827      2.2827   <open>
        2036            0      0.9840      0.9840   <method 'close' of 'file' objects>
        2036            0      0.0464      0.0464   <method 'read' of 'file' objects>
        5233            0      0.1985      0.0453   mercurial.repoview:161(changelog)
       10462            0      0.0791      0.0314       mercurial.changelog:133(tip)
        5233            0      0.0388      0.0176       mercurial.localrepo:26(__get__)
       10462            0      0.0250      0.0126       <len>
        5233            0      0.0059      0.0039       mercurial.repoview:112(filterrevs)
       10462            0      0.0029      0.0029       <hash>
        2034            0      0.0444      0.0444   <method 'seek' of 'file' objects>
        5340            0      0.0390      0.0390   mercurial.revlog:296(rev)
        2582            0      0.0371      0.0371   <zlib.decompress>
        3155            0      0.1963      0.0366   mercurial.context:202(__init__)
        3155            0      0.1238      0.0306       mercurial.repoview:161(changelog)
        3155            0      0.0261      0.0080       mercurial.changelog:183(rev)
        9465            0      0.0061      0.0061       <isinstance>
        1096            0      0.0023      0.0023       <binascii.unhexlify>
        4251            0      0.0014      0.0014       <len>
        2059            0      3.7341      0.0332   mercurial.changelog:270(read)
        2059            0      3.6304      0.0307       mercurial.revlog:907(revision)
        2057            0      0.0262      0.0137       mercurial.changelog:28(decodeextra)
        4118            0      0.0094      0.0094       <method 'split' of 'str' objects>
        4118            0      0.0270      0.0048       mercurial.encoding:61(tolocal)
        2059            0      0.0040      0.0040       <method 'index' of 'str' objects>
       10462            0      0.0791      0.0314   mercurial.changelog:133(tip)
       10462            0      0.0289      0.0207       mercurial.changelog:190(node)
       10462            0      0.0188      0.0091       <len>
       52433        20932      0.0478      0.0310   <len>
       20932            0      0.0221      0.0168       mercurial.revlog:262(__len__)
        2059            0      3.6304      0.0307   mercurial.revlog:907(revision)

real    0m4.361s
user    0m0.986s
sys     0m0.237s

After this change:

$ time hg --profile branches > /dev/null
   CallCount    Recursive     Total(s)    Inline(s) module:lineno(function)
        1069            0      1.1098      1.1098   <open>
        1063            0      0.4865      0.4865   <method 'close' of 'file' objects>
        4122            0      0.1811      0.0404   mercurial.repoview:161(changelog)
        8240            0      0.0712      0.0272       mercurial.changelog:133(tip)
        4122            0      0.0378      0.0177       mercurial.localrepo:26(__get__)
        8240            0      0.0221      0.0115       <len>
        4122            0      0.0057      0.0033       mercurial.repoview:112(filterrevs)
        8240            0      0.0025      0.0025       <hash>
        3029            0      0.1979      0.0371   mercurial.context:202(__init__)
        3029            0      0.1278      0.0310       mercurial.repoview:161(changelog)
        3029            0      0.0230      0.0081       mercurial.changelog:183(rev)
        9087            0      0.0061      0.0061       <isinstance>
        1096            0      0.0026      0.0026       <binascii.unhexlify>
        4125            0      0.0014      0.0014       <len>
        4229            0      0.0337      0.0337   mercurial.revlog:296(rev)
        1061            0      0.0296      0.0296   <method 'seek' of 'file' objects>
        1063            0      0.0292      0.0292   <method 'read' of 'file' objects>
        8240            0      0.0712      0.0272   mercurial.changelog:133(tip)
        8240            0      0.0271      0.0196       mercurial.changelog:190(node)
        8240            0      0.0169      0.0083       <len>
       40476        16488      0.0422      0.0271   <len>
       16488            0      0.0193      0.0152       mercurial.revlog:262(__len__)
        1342            0      0.0241      0.0241   <zlib.decompress>
        9445            0      0.0336      0.0224   mercurial.changelog:190(node)
        9445            0      0.0112      0.0112       mercurial.revlog:317(node)
        1074            0      1.9102      0.0224   mercurial.changelog:270(read)
        1074            0      1.8397      0.0202       mercurial.revlog:907(revision)
        1073            0      0.0187      0.0099       mercurial.changelog:28(decodeextra)
        2148            0      0.0061      0.0061       <method 'split' of 'str' objects>
        2148            0      0.0184      0.0034       mercurial.encoding:61(tolocal)

real    0m2.402s
user    0m0.735s
sys     0m0.177s