gitk: Fix some bugs with path limiting in the diff display

gitk: Fix some bugs with path limiting in the diff display

First, we weren't putting "--" between the ids and the paths in the
git diff-tree/diff-index/diff-files command, so if there was a tag
and a file with the same name, we could get an ambiguity in the
command.  This puts the "--" in to make it clear that the paths are
paths.

Secondly, this implements the path limiting for merge diffs as well
as the normal 2-way diffs.

Signed-off-by: Paul Mackerras <paulus@samba.org>