Merge branch 'ps/includeif-onbranch-cornercase-fix'

"git --git-dir=nowhere cmd" failed to properly notice that it wasn't in any repository while processing includeIf.onbranch configuration and instead crashed. * ps/includeif-onbranch-cornercase-fix: config: fix evaluating "onbranch" with nonexistent git dir t1305: exercise edge cases of "onbranch" includes