git-am: Mention --abort in usage string part of OPTIONS_SPEC

The three separate lines for --skip, --resolved and --abort
are merged into one so that it is easy to see that they're
alternative and related options.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -6,8 +6,7 @@ SUBDIRECTORY_OK=Yes
 OPTIONS_KEEPDASHDASH=
 OPTIONS_SPEC="\
 git am [options] [<mbox>|<Maildir>...]
-git am [options] --resolved
-git am [options] --skip
+git am [options] (--resolved | --skip | --abort)
 --
 d,dotest=       (removed -- do not use)
 i,interactive   run interactively