git-gui: Use prefix if blame is run in a subdirectory

git-gui: Use prefix if blame is run in a subdirectory

I think it was Andy Parkins who pointed out that git gui blame HEAD f
does not work if f is in a subdirectory and we are currently running
git-gui within that subdirectory.  This is happening because we did
not take the user's prefix into account when we computed the file
path in the repository.

We now assume the prefix as returned by rev-parse --show-prefix is
valid and we use that during the command line blame subcommand when
we apply the parameters.

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