am, rebase: fix arghelp syntax of --empty

Use parentheses and pipes to present alternatives in the argument help
for the --empty options of git am and git rebase, like in the rest of
the documentation.

While at it remove a stray use of the enum empty_action value
STOP_ON_EMPTY_COMMIT to indicate that no short option is present.
While it has a value of 0 and thus there is no user-visible change,
that enum is not meant to hold short option characters.  Hard-code 0,
like we do for other options without a short option.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

Documentation/git-rebase.txt

@@ -289,7 +289,7 @@ See also INCOMPATIBLE OPTIONS below.
 +
 See also INCOMPATIBLE OPTIONS below.
 
---empty={drop,keep,ask}::
+--empty=(drop|keep|ask)::
 	How to handle commits that are not empty to start and are not
 	clean cherry-picks of any upstream commit, but which become
 	empty after rebasing (because they contain a subset of already
@@ -695,7 +695,7 @@ be dropped automatically with `--no-keep-empty`).
 Similar to the apply backend, by default the merge backend drops
 commits that become empty unless `-i`/`--interactive` is specified (in
 which case it stops and asks the user what to do).  The merge backend
-also has an `--empty={drop,keep,ask}` option for changing the behavior
+also has an `--empty=(drop|keep|ask)` option for changing the behavior
 of handling commits that become empty.
 
 Directory rename detection

builtin/am.c

@@ -2419,7 +2419,7 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		{ OPTION_STRING, 'S', "gpg-sign", &state.sign_commit, N_("key-id"),
 		  N_("GPG-sign commits"),
 		  PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
-		OPT_CALLBACK_F(STOP_ON_EMPTY_COMMIT, "empty", &state.empty_type, "{stop,drop,keep}",
+		OPT_CALLBACK_F(0, "empty", &state.empty_type, "(stop|drop|keep)",
 		  N_("how to handle empty patches"),
 		  PARSE_OPT_NONEG, am_option_parse_empty),
 		OPT_HIDDEN_BOOL(0, "rebasing", &state.rebasing,

builtin/rebase.c

@@ -1147,7 +1147,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				 "instead of ignoring them"),
 			      1, PARSE_OPT_HIDDEN),
 		OPT_RERERE_AUTOUPDATE(&options.allow_rerere_autoupdate),
-		OPT_CALLBACK_F(0, "empty", &options, "{drop,keep,ask}",
+		OPT_CALLBACK_F(0, "empty", &options, "(drop|keep|ask)",
 			       N_("how to handle commits that become empty"),
 			       PARSE_OPT_NONEG, parse_opt_empty),
 		OPT_CALLBACK_F('k', "keep-empty", &options, NULL,