mergetool--lib: improve support for vimdiff-style tool variants

The merge tools vimdiff2, vimdiff3, gvimdiff2, gvimdiff3 and bc3 are all
variants of the main tools vimdiff and bc. They are implemented in the
main and a one-liner script that just sources it exist for each.

Allow variants ending in [0-9] to be correctly wired without the need
for such one-liners, so instead of 5 scripts, only 1 (gvimdiff) is
needed.

Signed-off-by: pudinha <rogi@skylittlesystem.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/vimdiff

@@ -46,3 +46,11 @@ translate_merge_tool_path() {
 exit_code_trustable () {
 	true
 }
+
+list_tool_variants () {
+	for prefix in '' g; do
+		for suffix in '' 2 3; do
+			echo "${prefix}vimdiff${suffix}"
+		done
+	done
+}