Use "-b", not "--branch", when issuing "hg log" in Phabricator

Summary:
Mercurial renamed "--only-branch" to "--branch" about two years ago. "-b" exists in both versions.

http://www.selenic.com/pipermail/mercurial-devel/2010-April/020469.html

We have a few other cases where we use features that exist only in recent Mercurial (notably, 'ancestors' in log) but we can work around this one easily.

Test Plan: Looked at a Mercurial repo in Diffusion, verified that "log -b" commands issued and that the output was correct.

Reviewers: btrahan, Makinde, ipalaus

Reviewed By: ipalaus

CC: aran

Maniphest Tasks: T1264

Differential Revision: https://secure.phabricator.com/D2533