phabricator: extract the logic to amend diff properties to a function
This will be needed on a separate code path when dealing with folding revisions.
And since we know that will involve adding multiple local commmits to the diff
properties instead of just one, restructure the logic slightly to allow it.
Differential Revision: https://phab.mercurial-scm.org/D8312
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.