mergetools: add description to all diff/merge tools

The output of `git mergetool --tool-help` and `git difftool --tool-help`
only showed the `alias` of each available merge/diff tool.

It is not always obvious what tool these `aliases` end up using (ex:
`opendiff` runs `FileMerge` and `bc` runs `Beyond Compare`).

This commit adds a short description to each of them to help the user
identify the `alias` they want.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/diffmerge

@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" >/dev/null 2>&1
 }
 
+diff_cmd_help () {
+	echo "Use DiffMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use DiffMerge (requires a graphical session)"
+}
+
 exit_code_trustable () {
 	true
 }