mergetool: Teach about submodules

mergetool: Teach about submodules

When the index has conflicted submodules, mergetool used to mildly
clobber the module, renaming it to mymodule.BACKUP.nnnn, then failing to
copy it non-recursively.

Recognize submodules and offer a resolution instead:

  Submodule merge conflict for 'Shared':
    {local}: submodule commit ad9f12e3e6205381bf2163a793d1e596a9e211d0
    {remote}: submodule commit f5893fb70ec5646efcd9aa643c5136753ac89253
  Use (l)ocal or (r)emote, or (a)bort?

Selecting a commit will stage it, but not update the submodule (as git
does had there been no conflict). Type changes are also supported,
should the path be a submodule on one side, and a file, symlink,
directory, or deleted on the other.

Signed-off-by: Jonathon Mah <me@JonathonMah.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>