mergetools: create mergetool_find_win32_cmd() helper function for winmerge

Signed-off-by: Jacob Nisnevich <jacob.nisnevich@gmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -372,3 +372,28 @@ get_merge_tool () {
 	fi
 	echo "$merge_tool"
 }
+
+mergetool_find_win32_cmd () {
+	executable=$1
+	sub_directory=$2
+
+	# Use $executable if it exists in $PATH
+	if type -p "$executable" >/dev/null 2>&1
+	then
+		printf '%s' "$executable"
+		return
+	fi
+
+	# Look for executable in the typical locations
+	for directory in $(env | grep -Ei '^PROGRAM(FILES(\(X86\))?|W6432)=' |
+		cut -d '=' -f 2- | sort -u)
+	do
+		if test -n "$directory" && test -x "$directory/$sub_directory/$executable"
+		then
+			printf '%s' "$directory/$sub_directory/$executable"
+			return
+		fi
+	done
+
+	printf '%s' "$executable"
+}