rev-parse: --abbrev-ref option to shorten ref name

This applies the shorten_unambiguous_ref function to the object name.
Default mode is controlled by core.warnAmbiguousRefs. Else it is given as
optional argument to --abbrev-ref={strict|loose}.

This should be faster than 'git for-each-ref --format="%(refname:short)" <ref>'
for single refs.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -84,6 +84,11 @@ OPTIONS
 	unfortunately named tag "master"), and show them as full
 	refnames (e.g. "refs/heads/master").
 
+--abbrev-ref[={strict|loose}]::
+	A non-ambiguous short name of the objects name.
+	The option core.warnAmbiguousRefs is used to select the strict
+	abbreviation mode.
+
 --all::
 	Show all refs found in `$GIT_DIR/refs`.