completion: find out supported merge strategies correctly

"git-merge" is a binary executable these days, and looking for assignment
to $all_strategies variable with grep/sed does not work well.

When asked for an unknown strategy, pre-1.6.0 and post-1.6.0 "git merge"
commands respectively say:

    $ $HOME/git-snap-v1.5.6.5/bin/git merge -s help
    available strategies are: recur recursive octopus resolve stupid ours subtree
    $ $HOME/git-snap-v1.6.0/bin/git merge -s help
    Could not find merge strategy 'help'.
    Available strategies are: recursive octopus resolve ours subtree.

both on their standard error stream.  We can use this to learn what
strategies are supported.

The sed script is written in such a way that it catches both old and new
message styles ("Available" vs "available", and the full stop at the end).
It also allows future versions of "git merge" to line-wrap the list of
strategies, and add extra comments, like this:

    $ $HOME/git-snap-v1.6.1/bin/git merge -s help
    Could not find merge strategy 'help'.
    Available strategies are: blame recursive octopus resolve ours
    subtree.
    Also you have custom strategies: theirs

    Make sure you spell strategy names correctly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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