git-gui: Combine blame groups only if commit and filename match

git-gui: Combine blame groups only if commit and filename match

Consecutive chunks of a file could come from the same commit, but
have different original file names.  Previously we would have put
them into a single group, but then the hyperlink would jump to only
one of the files, and the other would not be accessible.  Now we can
get to the other file too.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>