gitk bug: file select in the tree mode

Hi All!

There is a bug in gitk (e.g. 2.11.0):

1) Choose a repository with files in a subdir (git's repo for example).
2) `cd` to a subdir (e.g. `xdiff`).
3) Run `gitk`.
4) Select 'Tree' in the 'Patch / Tree' panel.
5) Select any file with 'Highlight this too' or 'Highlight this only'
(e.g `xmerge.c`).
6) See the short file name (`xmerge.c`) instead of the full path
(`xdiff/xmerge.c`) in the 'Find commit touching path:' edit field. No
commits touching the file can be found.