last-modified: new subcommand to show when files were last modified

Similar to git-blame(1), introduce a new subcommand
git-last-modified(1). This command shows the most recent modification to
paths in a tree. It does so by expanding the tree at a given commit,
taking note of the current state of each path, and then walking
backwards through history looking for commits where each path changed
into its final commit ID.

Based-on-patch-by: Jeff King <peff@peff.net>
Improved-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -607,6 +607,7 @@ builtin_sources = [
   'builtin/index-pack.c',
   'builtin/init-db.c',
   'builtin/interpret-trailers.c',
+  'builtin/last-modified.c',
   'builtin/log.c',
   'builtin/ls-files.c',
   'builtin/ls-remote.c',