smerge autolaunch integration for conflicts in git

[Eric S. Raymond]

I had a look at Rüdiger Sonderfeld's patch partially implementing
smerge launching for conflicted files in git.

Good news: Completing the patch was surprisingly easy. Rüdiger's
premises about vc-git's internals seem correct.  The only missing
piece was the resolve command the git backend should use when it
detects that there are no conflict markers remaining, and that is just
"git add" - no dark magic.  I have merged the implementation to trunk.

Bad news: I don't have a practical way to test this change beyond
verifying that the Lisp compiles - it could go pear-shaped when it
sees a live conflict.  Somebody for which this meets a real use case,
like Stefan, will have to try it.

Good news: I merged it anyway because if it breaks, it's not going to
do so seriously and should be easy to fix.  Multiple git adds on a
file are harmless.  The most likely failure is that the regexp for
detecting conflicts in a status listing is slightly off, in which case
the launch hook will simply not fire.

Bad news: I wanted to do this really elegantly, by pulling most of
the smerge launch logic into vc-git.el.  Alas, it's not possible without a
fairly large and disruptive refactoring (which probably shouldn't be
done before 24 release). Before even thinking further in that
direction I want to make sure this patch actually works.
-- 
                <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>

Men trained in arms from their infancy, and animated by the love of liberty,
will afford neither a cheap or easy conquest.
        -- From the Declaration of the Continental Congress, July 1775.