Introduce <branch>@{push} short-hand to denote the remote-tracking
branch that tracks the branch at the remote the <branch> would be
pushed to.

* jk/at-push-sha1:
  for-each-ref: accept "%(push)" format
  for-each-ref: use skip_prefix instead of starts_with
  sha1_name: implement @{push} shorthand
  sha1_name: refactor interpret_upstream_mark
  sha1_name: refactor upstream_mark
  remote.c: add branch_get_push
  remote.c: return upstream name from stat_tracking_info
  remote.c: untangle error logic in branch_get_upstream
  remote.c: report specific errors from branch_get_upstream
  remote.c: introduce branch_get_upstream helper
  remote.c: hoist read_config into remote_get_1
  remote.c: provide per-branch pushremote name
  remote.c: hoist branch.*.remote lookup out of remote_get_1
  remote.c: drop "remote" pointer from "struct branch"
  remote.c: refactor setup of branch->merge list
  remote.c: drop default_remote_name variable