gitk: Fix the display of files when filtered by path

gitk: Fix the display of files when filtered by path

Launching 'gitk -- .' or 'gitk -- ..\t' restricts the display to files
under the given directory but the file list is left empty. This is because
the path_filter function fails to match the filenames which are relative
to the working tree to the filter which is filessytem relative.
This solves the problem by making both names fully qualified filesystem
paths before performing the comparison.

Tested-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>