gitk: Fix a couple more bugs in the path limiting

gitk: Fix a couple more bugs in the path limiting

First, paths ending in a slash were not matching anything.  This fixes
path_filter to handle paths ending in a slash (such entries have to
match a directory, and can't match a file, e.g., foo/bar/ can't match
a plain file called foo/bar).

Secondly, clicking in the file list pane (bottom right) was broken
because $treediffs($ids) contained all the files modified by the
commit, not just those within the file list.  This fixes that too.

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