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>

contrib/completion/git-completion.bash

@@ -271,15 +271,17 @@ __git_merge_strategies ()
 		echo "$__git_merge_strategylist"
 		return
 	fi
-	sed -n "/^all_strategies='/{
-		s/^all_strategies='//
-		s/'//
+	git merge -s help 2>&1 |
+	sed -n -e '/[Aa]vailable strategies are: /,/^$/{
+		s/\.$//
+		s/.*://
+		s/^[ 	]*//
+		s/[ 	]*$//
 		p
-		q
-		}" "$(git --exec-path)/git-merge"
+	}'
 }
 __git_merge_strategylist=
-__git_merge_strategylist="$(__git_merge_strategies 2>/dev/null)"
+__git_merge_strategylist=$(__git_merge_strategies 2>/dev/null)
 
 __git_complete_file ()
 {