mergetools: support TortoiseGitMerge

TortoiseMerge.exe was ben renamed to TortoiseGitMerge.exe (starting
with 1.8.0) in order to make it clear that it has special support
for git, and prevent confusion with the TortoiseSVN TortoiseMerge
version.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/tortoisemerge

@@ -11,7 +11,16 @@ merge_cmd () {
 			-theirs:"$REMOTE" -merged:"$MERGED"
 		check_unchanged
 	else
-		echo "TortoiseMerge cannot be used without a base" 1>&2
+		echo "$merge_tool_path cannot be used without a base" 1>&2
 		return 1
 	fi
 }
+
+translate_merge_tool_path() {
+	if type tortoisegitmerge >/dev/null 2>/dev/null
+	then
+		echo tortoisegitmerge
+	else
+		echo tortoisemerge
+	fi
+}