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 ('#' or the core.commentChar config if set) followed by
two lines describing what it is for. The scissor line - which will not be
translated - 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;
also add one for a diff starting with a submodule log and another one for
proper handling of the comment char.

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