commit -v: strip diffs and submodule shortlogs from the commit message

When using the '-v' option of "git commit" the diff added to the commit
message temporarily for editing is stripped off after the user exited the
editor by searching for "\ndiff --git " and truncating the commmit message
there if it is found. But this approach has two problems: when the commit
message itself contains a line starting with "diff --git" it will be
truncated there prematurely. And when the "diff.submodule" setting is set
to "log", the diff may start with "Submodule <hash1>..<hash2>", which will
be left in the commit message while it shouldn't.

Fix that by introducing a special scissor separator line starting with the
comment character '#' followed by a line describing what it is for. The
scissor line is used to reliably detect the start of the diff so it can be
chopped off from the commit message, no matter what the user enters there.
Turn a known test failure fixed by this change into a successful test and
add another one for a diff starting with a submodule log.

Reported-by: Ari Pollak <ari@debian.org>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>