Even during a conflicted merge, "git blame $path" always meant to
blame uncommitted changes to the "working tree" version; make it
more useful by showing cleanly merged parts as coming from the other
branch that is being merged.

This incidentally fixes an unrelated problem on a case insensitive
filesystem, where "git blame MAKEFILE" run in a history that has
"Makefile" but not "MAKEFILE" did not say "No such file MAKEFILE in
HEAD" but pretended as if "MAKEFILE" was a newly added file.

* jc/maint-blame-no-such-path:
  blame: allow "blame file" in the middle of a conflicted merge
  blame $path: avoid getting fooled by case insensitive filesystems