make commit_tree a library function

Until now, this has been part of the commit-tree builtin.
However, it is already used by other builtins (like commit,
merge, and notes), and it would be useful to access it from
library code.

The check_valid helper has to come along, too, but is given
a more library-ish name of "assert_sha1_type".

Otherwise, the code is unchanged. There are still a few
rough edges for a library function, like printing the utf8
warning to stderr, but we can address those if and when they
come up as inappropriate.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -718,6 +718,8 @@ extern int has_loose_object_nonlocal(const unsigned char *sha1);
 
 extern int has_pack_index(const unsigned char *sha1);
 
+extern void assert_sha1_type(const unsigned char *sha1, enum object_type expect);
+
 extern const signed char hexval_table[256];
 static inline unsigned int hexval(unsigned char c)
 {