hash: add a function to lookup hash algorithm by length

There are some cases, such as the dumb HTTP transport and bundles, where
we can only determine the hash algorithm in use by the length of the
object IDs. Provide a function that looks up the algorithm by length.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hash.h

@@ -131,6 +131,8 @@ extern const struct git_hash_algo hash_algos[GIT_HASH_NALGOS];
 int hash_algo_by_name(const char *name);
 /* Identical, except based on the format ID. */
 int hash_algo_by_id(uint32_t format_id);
+/* Identical, except based on the length. */
+int hash_algo_by_length(int len);
 /* Identical, except for a pointer to struct git_hash_algo. */
 static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
 {

sha1-file.c

@@ -189,6 +189,14 @@ int hash_algo_by_id(uint32_t format_id)
 	return GIT_HASH_UNKNOWN;
 }
 
+int hash_algo_by_length(int len)
+{
+	int i;
+	for (i = 1; i < GIT_HASH_NALGOS; i++)
+		if (len == hash_algos[i].rawsz)
+			return i;
+	return GIT_HASH_UNKNOWN;
+}
 
 /*
  * This is meant to hold a *small* number of objects that you would