help: redirect to aliased commands for "git cmd --help"

As discussed in the thread for v1 of this patch [1] [2], this changes the
rules for "git foo --help" when foo is an alias.

(1) When invoked as "git help foo", we continue to print the "foo is
aliased to bar" message and nothing else.

(2) If foo is an alias for a shell command, print "foo is aliased to
!bar" as usual.

(3) Otherwise, print "foo is aliased to bar" to the standard error
stream, and then break the alias string into words and pretend as if
"git word[0] --help" were called.

Getting the man page for git-cherry-pick directly with "git cp --help"
is consistent with "--help" generally providing more comprehensive help
than "-h". Printing the alias definition to stderr means that in certain
cases (e.g. if help.format=web or if the pager uses an alternate screen
and does not clear the terminal), one has

'cp' is aliased to 'cherry-pick -n'

above the prompt when one returns to the terminal/quits the pager, which
is a useful reminder that using 'cp' has some flag implicitly set. There
are cases where this information vanishes or gets scrolled
away, but being printed to stderr, it should never hurt.

[1] https://public-inbox.org/git/20180926102636.30691-1-rv@rasmusvillemoes.dk/
[2] https://public-inbox.org/git/20180926184914.GC30680@sigill.intra.peff.net/

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -415,10 +415,38 @@ static const char *check_git_cmd(const char* cmd)
 
 	alias = alias_lookup(cmd);
 	if (alias) {
+		const char **argv;
+		int count;
+
+		/*
+		 * handle_builtin() in git.c rewrites "git cmd --help"
+		 * to "git help --exclude-guides cmd", so we can use
+		 * exclude_guides to distinguish "git cmd --help" from
+		 * "git help cmd". In the latter case, or if cmd is an
+		 * alias for a shell command, just print the alias
+		 * definition.
+		 */
+		if (!exclude_guides || alias[0] == '!') {
 			printf_ln(_("'%s' is aliased to '%s'"), cmd, alias);
 			free(alias);
 			exit(0);
 		}
+		/*
+		 * Otherwise, we pretend that the command was "git
+		 * word0 --help". We use split_cmdline() to get the
+		 * first word of the alias, to ensure that we use the
+		 * same rules as when the alias is actually
+		 * used. split_cmdline() modifies alias in-place.
+		 */
+		fprintf_ln(stderr, _("'%s' is aliased to '%s'"), cmd, alias);
+		count = split_cmdline(alias, &argv);
+		if (count < 0)
+			die(_("bad alias.%s string: %s"), cmd,
+			    split_cmdline_strerror(count));
+		free(argv);
+		UNLEAK(alias);
+		return alias;
+	}
 
 	if (exclude_guides)
 		return help_unknown_cmd(cmd);