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