reset: give better reflog messages

reset: give better reflog messages

The reset command creates its reflog entry from argv.
However, it does so after having run parse_options, which
means the only thing left in argv is any non-option
arguments. Thus you would end up with confusing reflog
entries like:

  $ git reset --hard HEAD^
  $ git reset --soft HEAD@{1}
  $ git log -2 -g --oneline
  8e46cad HEAD@{0}: HEAD@{1}: updating HEAD
  1eb9486 HEAD@{1}: HEAD^: updating HEAD

However, we must also consider that some scripts may set
GIT_REFLOG_ACTION before calling reset, and we need to show
their reflog action (with our text appended). For example:

  rebase -i (squash): updating HEAD

On top of that, we also set the ORIG_HEAD reflog action
(even though it doesn't generally exist). In that case, the
reset argument is somewhat meaningless, as it has nothing to
do with what's in ORIG_HEAD.

This patch changes the reset reflog code to show:

  $GIT_REFLOG_ACTION: updating {HEAD,ORIG_HEAD}

as before, but only if GIT_REFLOG_ACTION is set. Otherwise,
show:

   reset: moving to $rev

for HEAD, and:

   reset: updating ORIG_HEAD

for ORIG_HEAD (this is still somewhat superfluous, since we
are in the ORIG_HEAD reflog, obviously, but at least we now
mention which command was used to update it).

While we're at it, we can clean up the code a bit:

 - Use strbufs to make the message.

 - Use the "rev" parameter instead of showing all options.
   This makes more sense, since it is the only thing
   impacting the writing of the ref.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>