gitk: add external diff file rename detection

gitk: add external diff file rename detection

If a file is renamed between commits and an external diff is started
through gitk on the original or the renamed file name,
gitk is unable to open the renamed file in the external diff editor.
It fails to fetch the renamed file from git, because it fetches it
using its original path in contrast to using the renamed path of the
file.
Detect the rename and open the external diff with the original and
the renamed file instead of no file (fetch the renamed file path and
name from git) no matter if the original or the renamed file is
selected in gitk.

Signed-off-by: Tobias Boesch <tobias.boesch@miele.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>