completion: fix zsh alias listing for subsection aliases

The zsh completion function __git_zsh_cmd_alias() uses 'git config
--get-regexp' to enumerate aliases and then strips the "alias." prefix
from each key. For subsection-style aliases (alias.name.command), this
leaves "name.command" as the completion candidate instead of just
"name".

The bash completion does not have this problem because it goes through
'git --list-cmds=alias', which calls list_aliases() in C and already
handles both alias syntaxes correctly. However, zsh needs both the
alias name and its value for descriptive completion, which
--list-cmds=alias does not provide.

Add a hidden --aliases-for-completion option to 'git help', following
the existing --config-for-completion pattern. It outputs NUL-separated
"name\nvalue" pairs using list_aliases(), which correctly resolves both
the traditional (alias.name) and subsection (alias.name.command)
formats. Update __git_zsh_cmd_alias() to use it.

Signed-off-by: Jonatan Holmgren <jonatan@jontes.page>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jonatan-holmgren

builtin/help.c

@@ -54,6 +54,7 @@ static enum help_action {
 	HELP_ACTION_DEVELOPER_INTERFACES,
 	HELP_ACTION_CONFIG_FOR_COMPLETION,
 	HELP_ACTION_CONFIG_SECTIONS_FOR_COMPLETION,
+	HELP_ACTION_ALIASES_FOR_COMPLETION,
 } cmd_mode;
 
 static char *html_path;
@@ -90,6 +91,8 @@ static struct option builtin_help_options[] = {
 		    HELP_ACTION_CONFIG_FOR_COMPLETION, PARSE_OPT_HIDDEN),
 	OPT_CMDMODE_F(0, "config-sections-for-completion", &cmd_mode, "",
 		    HELP_ACTION_CONFIG_SECTIONS_FOR_COMPLETION, PARSE_OPT_HIDDEN),
+	OPT_CMDMODE_F(0, "aliases-for-completion", &cmd_mode, "",
+		    HELP_ACTION_ALIASES_FOR_COMPLETION, PARSE_OPT_HIDDEN),
 
 	OPT_END(),
 };
@@ -691,6 +694,16 @@ int cmd_help(int argc,
 			       help_format);
 		list_config_help(SHOW_CONFIG_SECTIONS);
 		return 0;
+	case HELP_ACTION_ALIASES_FOR_COMPLETION: {
+		struct string_list alias_list = STRING_LIST_INIT_DUP;
+		opt_mode_usage(argc, "--aliases-for-completion", help_format);
+		list_aliases(&alias_list);
+		for (size_t i = 0; i < alias_list.nr; i++)
+			printf("%s%c%s%c", alias_list.items[i].string, '\n',
+			       (char *)alias_list.items[i].util, '\0');
+		string_list_clear(&alias_list, 1);
+		return 0;
+	}
 	case HELP_ACTION_CONFIG:
 		opt_mode_usage(argc, "--config", help_format);
 		setup_pager(the_repository);

contrib/completion/git-completion.zsh

@@ -202,7 +202,7 @@ __git_zsh_cmd_common ()
 __git_zsh_cmd_alias ()
 {
 	local -a list
-	list=(${${(0)"$(git config -z --get-regexp '^alias\.*')"}#alias.})
+	list=(${(0)"$(git help --aliases-for-completion)"})
 	list=(${(f)"$(printf "%s:alias for '%s'\n" ${(f@)list})"})
 	_describe -t alias-commands 'aliases' list && _ret=0
 }