Merge branch 'ps/show-ref'

Merge branch 'ps/show-ref'

Teach "git show-ref" a mode to check the existence of a ref.

* ps/show-ref:
  t: use git-show-ref(1) to check for ref existence
  builtin/show-ref: add new mode to check for reference existence
  builtin/show-ref: explicitly spell out different modes in synopsis
  builtin/show-ref: ensure mutual exclusiveness of subcommands
  builtin/show-ref: refactor options for patterns subcommand
  builtin/show-ref: stop using global vars for `show_one()`
  builtin/show-ref: stop using global variable to count matches
  builtin/show-ref: refactor `--exclude-existing` options
  builtin/show-ref: fix dead code when passing patterns
  builtin/show-ref: fix leaking string buffer
  builtin/show-ref: split up different subcommands
  builtin/show-ref: convert pattern to a local variable