doc/merge-config: list ecmerge as a built-in merge tool

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-config.txt

@@ -22,7 +22,8 @@ merge.stat::
 merge.tool::
 	Controls which merge resolution program is used by
 	linkgit:git-mergetool[1].  Valid built-in values are: "kdiff3",
-	"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff", and
+	"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff",
+	"ecmerge" and
 	"opendiff".  Any other value is treated is custom merge tool
 	and there must be a corresponding mergetool.<tool>.cmd option.