Implement automatic fast-forward merge for submodules

This implements a simple merge strategy for submodule hashes. We check
whether one side of the merge candidates is already contained in the
other and then merge automatically.

If both sides contain changes we search for a merge in the submodule.
In case a single one exists we check that out and suggest it as the
merge resolution. A list of candidates is returned when we find multiple
merges that contain both sides of the changes.

This is useful for a workflow in which the developers can publish topic
branches in submodules and a separate maintainer merges them. In case
the developers always wait until their branch gets merged before tracking
them in the superproject all merges of branches that contain submodule
changes will be resolved automatically. If developers choose to track
their feature branch the maintainer might get a conflict but git will
search the submodule for a merge and suggest it/them as a resolution.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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