reset: improve mixed reset error message when in a bare repo

When running a "git reset --mixed" in a bare repository, the
message displayed is something like:

fatal: This operation must be run in a work tree
fatal: Could not reset index file to revision 'HEAD^'.

This message is a little bit misleading because a mixed reset is
ok in a git directory, so it is not absolutely needed to run it in
a work tree.

So this patch improves upon the above by changing the message to:

fatal: mixed reset is not allowed in a bare repository

And if "git reset" is ever sped up by using unpack_tree() directly
(instead of execing "git read-tree"), this patch will also make
sure that a mixed reset is still disallowed in a bare repository.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-reset.c

@@ -291,6 +291,10 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		die("%s reset requires a work tree",
 		    reset_type_names[reset_type]);
 
+	if (reset_type == MIXED && is_bare_repository())
+		die("%s reset is not allowed in a bare repository",
+		    reset_type_names[reset_type]);
+
 	/* Soft reset does not touch the index file nor the working tree
 	 * at all, but requires them in a good order.  Other resets reset
 	 * the index file to the tree object we are switching to. */