New config push.default to decide default behavior for push

When "git push" is not told what refspecs to push, it pushes all matching
branches to the current remote.  For some workflows this default is not
useful, and surprises new users.  Some have even found that this default
behaviour is too easy to trigger by accident with unwanted consequences.

Introduce a new configuration variable "push.default" that decides what
action git push should take if no refspecs are given or implied by the
command line arguments or the current remote configuration.

Possible values are:

  'nothing'  : Push nothing;
  'matching' : Current default behaviour, push all branches that already
               exist in the current remote;
  'tracking' : Push the current branch to whatever it is tracking;
  'current'  : Push the current branch to a branch of the same name,
               i.e. HEAD.

Signed-off-by: Finn Arne Gangstad <finnag@pvv.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -42,6 +42,7 @@ enum safe_crlf safe_crlf = SAFE_CRLF_WARN;
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;
+enum push_default_type push_default = PUSH_DEFAULT_UNSPECIFIED;
 
 /* Parallel index stat data preload? */
 int core_preload_index = 0;