git.c: handle_alias: prepend alias info when first argument is -h

Most git commands respond to -h anywhere in the command line, or at
least as a first and lone argument, by printing the usage
information. For aliases, we can provide a little more information that
might be useful in interpreting/understanding the following output by
prepending a line telling that the command is an alias, and for what.

When one invokes a simple alias, such as "cp = cherry-pick"
with -h, this results in

$ git cp -h
'cp' is aliased to 'cherry-pick'
usage: git cherry-pick [<options>] <commit-ish>...
...

When the alias consists of more than one word, this provides the
additional benefit of informing the user which options are implicit in
using the alias, e.g. with "cp = cherry-pick -n":

$ git cp -h
'cp' is aliased to 'cherry-pick -n'
usage: git cherry-pick [<options>] <commit-ish>...
...

For shell commands, we cannot know how it responds to -h, but printing
this line to stderr should not hurt, and can help in figuring out what
is happening in a case like

$ git sc -h
'sc' is aliased to '!somecommand'
somecommand: invalid option '-h'

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

git.c

@@ -318,6 +318,9 @@ static int handle_alias(int *argcp, const char ***argv)
 	alias_command = (*argv)[0];
 	alias_string = alias_lookup(alias_command);
 	if (alias_string) {
+		if (*argcp > 1 && !strcmp((*argv)[1], "-h"))
+			fprintf_ln(stderr, _("'%s' is aliased to '%s'"),
+				   alias_command, alias_string);
 		if (alias_string[0] == '!') {
 			struct child_process child = CHILD_PROCESS_INIT;
 			int nongit_ok;