gitk: Fix file highlight when run in subdirectory

gitk: Fix file highlight when run in subdirectory

The "highlight this only" and "highlight this too" commands in gitk
add the path relative to $GIT_WORK_TREE to the "Find" input box. When
the search (using git-diff-tree) is run, the paths are used
unmodified, except for some shell escaping. Since the search is run
from gitk's working directory, no commits matching the paths will be
found if gitk was started in a subdirectory.

Make the paths passed to git-diff-tree relative to gitk's working
directory instead of being relative to $GIT_WORK_TREE. If, however,
gitk is run outside of the working directory (e.g. with $GIT_WORK_TREE
set), we still need to use the path relative to $GIT_WORK_TREE.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>