show-branch: show all local heads when only giving one rev along --topics

show-branch: show all local heads when only giving one rev along --topics

"git show-branch --topics <rev> <revs>..." displays ancestry graph, only
considering commits that are in all given revs, except the first one.

"git show-branch" displays ancestry graph for all local branches.

Unfortunately, "git show-branch --topics <rev>" only prints out the rev
info for the given rev, and nothing else, e.g.:

  $ git show-branch --topics origin/master
  [origin/master] Sync with 2.3.3

While there is an option to add all remote-tracking branches (-r), and
another to add all local+remote branches (-a), there is no option to add
only local branches. Adding such an option could be considered, but a
user would likely already expect that the above command line considers
the lack of rev other than for --topics as meaning all local branches,
like when there is no argument at all.

Moreover, when using -r and -a along with --topics, the first local or
remote-tracking branch, depending on alphabetic order is used instead of
the one given after --topics (any rev given on the command line is
actually simply ignored when either -r or -a is given). And if no rev is
given at all, the fact that the first alphetical branch is the base of
topics is probably not expected by users (Maybe --topics should always
require one rev on the command line?)

This change makes
  "show-branch --topics $rev"
act as
  "show-branch --topics $rev $(git for-each-ref refs/heads
                               --format='%(refname:short)')"

  "show-branch -r --topics $rev ..."
act as
  "show-branch --topics $rev ... $(git for-each-ref refs/remotes
                                   --format='%(refname:short)')"
instead of
  "show-branch --topics $(git for-each-ref refs/remotes
                          --format='%(refname:short)')"

and
  "show-branch -a --topics $rev ..."
act as
  "show-branch --topics $rev ... $(git for-each-ref refs/heads refs/remotes
                                   --format='%(refname:short)')"
instead of
  "show-branch --topics $(git for-each-ref refs/heads refs/remotes
                          --format='%(refname:short)')"

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>