rhg: faster hg cat when many files are requested
With this patch I'm seeing a ~39ms improvement (220ms -> 181ms) when
running [hg cat] on ~220 files in a ~260k-file repo.
The timing for [hg cat] on an individual file becomes slightly worse
(losing 5ms: 145ms -> 150ms).
A follow-up commit is intended to improve that.
Differential Revision: https://phab.mercurial-scm.org/D11615
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).