builtin-branch: improve output when displaying remote branches

builtin-branch: improve output when displaying remote branches

When encountering a symref (typically refs/remotes/<remote>/HEAD),
display the ref target.

When displaying local and remote branches, prefix the remote branch
names with "remotes/" to make the remote branches clear from the local
branches. If displaying only the remote branches, the prefix is not
shown since it would be redundant.

Sample output:

$ git branch
  foo -> master
* master
  rather-long-branch-name

$ git branch -v
  foo                     -> master
* master                  51cecb2 initial
  rather-long-branch-name 51cecb2 initial

$ git branch -v --no-abbrev
  foo                     -> master
* master                  51cecb2dbb1a1902bb4df79b543c8f951ee59d83 initial
  rather-long-branch-name 51cecb2dbb1a1902bb4df79b543c8f951ee59d83 initial

$ git branch -r
  frotz/HEAD -> frotz/master
  frotz/master
  origin/HEAD -> origin/master
  origin/UNUSUAL -> refs/heads/master
  origin/master

$ git branch -a
  foo -> master
* master
  rather-long-branch-name
  remotes/frotz/HEAD -> frotz/master
  remotes/frotz/master
  remotes/origin/HEAD -> origin/master
  remotes/origin/UNUSUAL -> refs/heads/master
  remotes/origin/master

$ git branch -rv
  frotz/HEAD     -> frotz/master
  frotz/master   e1d8130 added file2
  origin/HEAD    -> origin/master
  origin/UNUSUAL -> refs/heads/master
  origin/master  e1d8130 added file2

$ git branch -av
  foo                     -> master
* master                  51cecb2 initial
  rather-long-branch-name 51cecb2 initial
  remotes/frotz/HEAD      -> frotz/master
  remotes/frotz/master    e1d8130 added file2
  remotes/origin/HEAD     -> origin/master
  remotes/origin/UNUSUAL  -> refs/heads/master
  remotes/origin/master   e1d8130 added file2

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>