branch -d: avoid repeated symref resolution

If a repository gets in a broken state with too much symref nesting,
it cannot be repaired with "git branch -d":

 $ git symbolic-ref refs/heads/nonsense refs/heads/nonsense
 $ git branch -d nonsense
 error: branch 'nonsense' not found.

Worse, "git update-ref --no-deref -d" doesn't work for such repairs
either:

 $ git update-ref -d refs/heads/nonsense
 error: unable to resolve reference refs/heads/nonsense: Too many levels of symbolic links

Fix both by teaching resolve_ref_unsafe a new RESOLVE_REF_NO_RECURSE
flag and passing it when appropriate.

Callers can still read the value of a symref (for example to print a
message about it) with that flag set --- resolve_ref_unsafe will
resolve one level of symrefs and stop there.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -177,10 +177,12 @@ extern int peel_ref(const char *refname, unsigned char *sha1);
  * ref_transaction_create(), etc.
  * REF_NODEREF: act on the ref directly, instead of dereferencing
  *              symbolic references.
+ * REF_DELETING: tolerate broken refs
  *
  * Flags >= 0x100 are reserved for internal use.
  */
 #define REF_NODEREF	0x01
+#define REF_DELETING	0x02
 /*
  * This function sets errno to something meaningful on failure.
  */