git-gui: stop using deprecated merge syntax

Starting with v2.5.0 git merge can handle FETCH_HEAD internally and
warns when it's called like 'git merge <message> HEAD <commit>' because
that syntax is deprecated.  Use this feature in git-gui and get rid of
that warning.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Tested-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lib/merge.tcl

@@ -112,12 +112,7 @@ method _start {} {
 	close $fh
 	set _last_merged_branch $branch
 
-	set cmd [list git]
-	lappend cmd merge
-	lappend cmd --strategy=recursive
-	lappend cmd [git fmt-merge-msg <[gitdir FETCH_HEAD]]
-	lappend cmd HEAD
-	lappend cmd $name
+	set cmd [list git merge --strategy=recursive FETCH_HEAD]
 
 	ui_status [mc "Merging %s and %s..." $current_branch $stitle]
 	set cons [console::new [mc "Merge"] "merge $stitle"]