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:

• [DO NOT MERGE, HELP NEEDED] pkg_uptodate experiment pkgcore/pkgcore#315
• https://bugs.gentoo.org/788394
• ebuilds with EGIT_COMMIT_DATE set should be skipped #13
• Cannot distinguish between EGIT_COMMIT and ${PN}_LIVE_COMMIT #5