builtin-remote: new show output style for push refspecs

builtin-remote: new show output style for push refspecs

The existing output of "git remote show <remote>" with respect to push
ref specs is basically just to show the raw refspec. This patch teaches
the command to interpret the refspecs and show how each branch will be
pushed to the destination. The output gives the user an idea of what
"git push" should do if it is run w/o any arguments.

Example new output:

1a. Typical output with no push refspec (i.e. matching branches only)

$ git remote show origin
* remote origin
  [...]
  Local refs configured for 'git push':
    master pushes to master (up to date)
    next   pushes to next   (local out of date)

1b. Same as above, w/o querying the remote:

$ git remote show origin -n
* remote origin
  [...]
  Local ref configured for 'git push' (status not queried):
    (matching) pushes to (matching)

2a. With a forcing refspec (+), and a new topic
    (something like push = refs/heads/*:refs/heads/*):

$ git remote show origin
* remote origin
  [...]
  Local refs configured for 'git push':
    master     pushes to master    (fast forwardable)
    new-topic  pushes to new-topic (create)
    next       pushes to next      (local out of date)
    pu         forces to pu        (up to date)

2b. Same as above, w/o querying the remote

$ git remote show origin -n
* remote origin
  [...]
  Local refs configured for 'git push' (status not queried):
    master     pushes to master
    new-topic  pushes to new-topic
    next       pushes to next
    pu         forces to pu

3. With a remote configured as a mirror:

* remote backup
  [...]
  Local refs will be mirrored by 'git push'

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