mergetools: add winmerge as a builtin tool

Add a winmerge scriptlet with the commands described in [1] so
that users can use winmerge without needing to perform any
additional configuration.

[1] http://thread.gmane.org/gmane.comp.version-control.git/268631

Helped-by: Philip Oakley <philipoakley@iee.org>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Sebastian Schuberth <sschuberth@gmail.com>
Helped-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/winmerge

@@ -0,0 +1,36 @@
+diff_cmd () {
+	"$merge_tool_path" -u -e "$LOCAL" "$REMOTE"
+	return 0
+}
+
+merge_cmd () {
+	# mergetool.winmerge.trustExitCode is implicitly false.
+	# touch $BACKUP so that we can check_unchanged.
+	touch "$BACKUP"
+	"$merge_tool_path" -u -e -dl Local -dr Remote \
+		"$LOCAL" "$REMOTE" "$MERGED"
+	check_unchanged
+}
+
+translate_merge_tool_path() {
+	# Use WinMergeU.exe if it exists in $PATH
+	if type -p WinMergeU.exe >/dev/null 2>&1
+	then
+		printf WinMergeU.exe
+		return
+	fi
+
+	# Look for WinMergeU.exe in the typical locations
+	winmerge_exe="WinMerge/WinMergeU.exe"
+	for directory in $(env | grep -Ei '^PROGRAM(FILES(\(X86\))?|W6432)=' |
+		cut -d '=' -f 2- | sort -u)
+	do
+		if test -n "$directory" && test -x "$directory/$winmerge_exe"
+		then
+			printf '%s' "$directory/$winmerge_exe"
+			return
+		fi
+	done
+
+	printf WinMergeU.exe
+}