checkout -b <name>: correctly detect existing branch

When create a new branch, we fed "refs/heads/<proposed name>" as a string
to get_sha1() and expected it to fail when a branch already exists.

The right way to check if a ref exists is to check with resolve_ref().

A naïve solution that might appear attractive but does not work is to
forbid slashes in get_describe_name() but that will not work. A describe
name is is in the form of "ANYTHING-g<short sha1>", and that ANYTHING part
comes from a original tag name used in the repository the user ran the
describe command. A sick user could have a confusing hierarchical tag
whose name is "refs/heads/foobar" (stored as refs/tags/refs/heads/foobar")
to generate a describe name "refs/heads/foobar-6-g02ac983", and we should
be able to use that name to refer to the object whose name is 02ac983.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -46,6 +46,7 @@ extern void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refn
  */
 extern void add_extra_ref(const char *refname, const unsigned char *sha1, int flags);
 extern void clear_extra_refs(void);
+extern int ref_exists(char *);
 
 extern int peel_ref(const char *, unsigned char *);