changectx: increase perf of walk function

When running 'hg cat -r . <file>' it was doing an expensive ctx.walk(m) which
applied the regex to every file in the manifest.

This changes changectx.walk to iterate over just the files in the regex, if no
other patterns are specified. This cuts hg cat time by 50% in our repo and
probably benefits a few other commands as well.
cat: increase perf when catting single files

Special case the single file case in hg cat. This allows us to avoid
parsing the manifest, which shaves 15% off hg cat perf. This is worth
it, since automation often uses hg cat for retrieving single files.
diff: search beyond ancestor when detecting renames

This removes an optimization that was introduced in 91eb4512edd0 but was too
aggressive - as indicated by how it changed test-mq-merge.t .

We are walking filelogs to find copy sources and we can thus not be sure to hit
the base revision and find the renamed file there - it could also be in the
first ancestor of the base ... in the filelog.

We are walking the filelog and can thus not easily know when we hit the first
ancestor of the base revision and which filename to look for there. Instead, we
use _findlimit like mergecopies do: The lower bound for how far we have to go
is found from the lowest changelog revision that is an ancestor of only one of
the compared revisions. Any filelog ancestor with a revision number lower than
that revision will be the ancestor of both compared revisions, and there is
thus no reason to go further back than that.
largefiles: drop redundant special handling of merges of renames

It is unclear what cases this was supposed to cover but it do no longer seem
relevant.
largefiles: don't try to explain rename history before prompt for conflicts

Before it tried to explain the exact situation when merging moved largefiles.
That do not happen for normal merges and is not more relevant for largefiles
than for normal files. It is unneeded complexity - remove it.
largefiles: show hashes before prompting for conflict resolution

The largefile hashes are mostly an implementation detail, but they are "leaked"
in several places anyway, and showing the hashes is better than not giving the
user any information about the options in the prompt.

The hashes are long, but it is largefile hashes and it would thus be confusing
to shorten them.
largefiles: stylistic cleanup of filemerge
phases: add a formal note that hash of secret changeset may leak out

For technical reason (discovery, obsolescence marker) the hash of secret
changeset are communicated outside of your repo. We clarifie that in the help so
that people does not used hash of secret changeset as nuclear launch code.
record: use commands.diffwsopts instead of ad-hoc diffopts

The record extension is writing its own version of commands.diffwsopts
which is identical to commands.diffwsopts. Based on the principle that
code duplication increases maintenance burden, this patch removes
record's ad-hoc diffopts in favour of commands.diffwsopts
record: re-enable whitespace-ignoring options

It looks like somewhere down the line, patch.diffopts changed the
names of the options that it recognises, but record.recordfunc wasn't
updated to the new names. Instead of trying to write down names at
all, we now use whatever names are provided in commands.diffwsopts and
pass that along to patch.diffopts, along with a couple of custom
options