gitk: Display dirty submodules correctly

gitk: Display dirty submodules correctly

Since recently "git diff --submodule" prints out extra lines when the
submodule contains untracked or modified files. Show all those lines of
one submodule under the same header.

Also for newly added or removed submodules the submodule name contained
trailing garbage because the extraction of the name was not done right.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>