git-gui: Enable resolution of merge conflicts.

If a file has a merge conflict (index state = U) the user will need to
run update-index on that file to resolve all stages down to stage 0,
by including the file in the working directory.

Like core Git we'll just trust the user that their resolution is
correct, and that they didn't just include the file into the commit
while merge conflicts still exist within the file.

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