git-gui: Generate blame on uncommitted working tree file

git-gui: Generate blame on uncommitted working tree file

If the user doesn't give us a revision parameter to our blame
subcommand then we can generate blame against the working tree
file by passing the file path off to blame with the --contents
argument.  In this case we cannot obtain the contents of the
file from the ODB; instead we must obtain the contents by
reading the working directory file as-is.

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