git-remote: show all remotes with "git remote show"

Many other commands use the "no arguments" form to show a
list (e.g., git-branch, git-tag). While we did show all
remotes for just "git remote", we displayed a usage error
for "git remote show" with no arguments. This is
counterintuitive, since by giving it _more_ information, we
get _less_ result.

The usage model can now be thought of as:

  - "git remote show <remote>": show a remote
  - "git remote show": show all remotes
  - "git remote": assume "show"; i.e., shorthand for "git remote show"

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -19,6 +19,8 @@ static const char * const builtin_remote_usage[] = {
 
 static int verbose;
 
+static int show_all(void);
+
 static inline int postfixcmp(const char *string, const char *postfix)
 {
 	int len1 = strlen(string), len2 = strlen(postfix);
@@ -380,8 +382,11 @@ static int show_or_prune(int argc, const char **argv, int prune)
 
 	argc = parse_options(argc, argv, options, builtin_remote_usage, 0);
 
-	if (argc < 1)
+	if (argc < 1) {
+		if (!prune)
+			return show_all();
 		usage_with_options(builtin_remote_usage, options);
+	}
 
 	memset(&states, 0, sizeof(states));
 	for (; argc; argc--, argv++) {