Move mercurial commit metadata parsing into a LowLevel query

Summary: Ref T4195. Same as D7793, but for mercurial. (As usual, SVN needs some goofy nonsense instead, so the next diff will just make this field work.)

Test Plan: Ran `reparse.php` on Git and Mercurial commits, var_dump'd the output and it looked correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7795
Fix loop in svnserve workflow for large binaries

Summary: If you push a large binary and the data crosses multiple data frames, we can end up in a loop in the parser.

Test Plan:
After this change, I was able to push a 95MB binary in 7s, which seems reasonable:

  >>> orbital ~/repos/INIS $ svn st
  A       large2.bin
  >>> orbital ~/repos/INIS $ ls -alh
  total 390648
  drwxr-xr-x   6 epriestley  admin   204B Dec 18 17:14 .
  drwxr-xr-x  98 epriestley  admin   3.3K Dec 16 11:19 ..
  drwxr-xr-x   7 epriestley  admin   238B Dec 18 17:14 .svn
  -rw-r--r--   1 epriestley  admin    80B Dec 18 15:07 README
  -rw-r--r--   1 epriestley  admin    95M Dec 18 16:53 large.bin
  -rw-r--r--   1 epriestley  admin    95M Dec 18 17:14 large2.bin
  >>> orbital ~/repos/INIS $ time svn commit -m 'another large binary'
  Adding  (bin)  large2.bin
  Transmitting file data .
  Committed revision 25.

  real	0m7.215s
  user	0m5.327s
  sys	0m0.407s
  >>> orbital ~/repos/INIS $

There may be room to improve this by using `PhutilRope`.

Reviewers: wrotte, btrahan, wotte

Reviewed By: wotte

CC: aran

Differential Revision: https://secure.phabricator.com/D7798