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

Author: gitster

Documentation/git-show-ref.txt

@@ -8,10 +8,14 @@ git-show-ref - List references in a local repository
 SYNOPSIS
 --------
 [verse]
-'git show-ref' [-q | --quiet] [--verify] [--head] [-d | --dereference]
+'git show-ref' [--head] [-d | --dereference]
 	     [-s | --hash[=<n>]] [--abbrev[=<n>]] [--tags]
 	     [--heads] [--] [<pattern>...]
+'git show-ref' --verify [-q | --quiet] [-d | --dereference]
+	     [-s | --hash[=<n>]] [--abbrev[=<n>]]
+	     [--] [<ref>...]
 'git show-ref' --exclude-existing[=<pattern>]
+'git show-ref' --exists <ref>
 
 DESCRIPTION
 -----------
@@ -27,6 +31,10 @@ The `--exclude-existing` form is a filter that does the inverse. It reads
 refs from stdin, one ref per line, and shows those that don't exist in
 the local repository.
 
+The `--exists` form can be used to check for the existence of a single
+references. This form does not verify whether the reference resolves to an
+actual object.
+
 Use of this utility is encouraged in favor of directly accessing files under
 the `.git` directory.
 
@@ -62,6 +70,12 @@ OPTIONS
 	Aside from returning an error code of 1, it will also print an error
 	message if `--quiet` was not specified.
 
+--exists::
+
+	Check whether the given reference exists. Returns an exit code of 0 if
+	it does, 2 if it is missing, and 1 in case looking up the reference
+	failed with an error other than the reference being missing.
+
 --abbrev[=<n>]::
 
 	Abbreviate the object name.  When using `--hash`, you do
@@ -70,8 +84,8 @@ OPTIONS
 -q::
 --quiet::
 
-	Do not print any results to stdout. When combined with `--verify`, this
-	can be used to silently check if a reference exists.
+	Do not print any results to stdout. Can be used with `--verify` to
+	silently check if a reference exists.
 
 --exclude-existing[=<pattern>]::