spurious rebuilds caused becuase EGIT_OVERRIDE_BRANCH_* and friends are ignored

[tgbugs]

I have a number of repos where I set EGIT_OVERRIDE_BRANCH_ and as a result s-l-r detects that the repo has been "updated" when in fact it is just seeing the master branch commit instead of the override branch commit.

[thesamesam]

See also:

• https://github.com/pkgcore/pkgcore/pull/315
• https://bugs.gentoo.org/788394
• https://github.com/projg2/smart-live-rebuild/issues/13
• https://github.com/projg2/smart-live-rebuild/issues/5