Add "unpack_sha1_header()" helper function

It's for people who aren't necessarily interested in the whole
unpacked file, but do want to know the header information (size,
type, etc..)

For example, the delta code can use this to figure out whether
an object is already a delta object, and what it is a delta
against, without actually bothering to unpack all of the actual
data in the delta.

cache.h

@@ -151,6 +151,7 @@ extern char *sha1_file_name(const unsigned char *sha1);
 
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern void * map_sha1_file(const unsigned char *sha1, unsigned long *size);
+extern int unpack_sha1_header(z_stream *stream, void *map, unsigned long mapsize, void *buffer, unsigned long size);
 extern void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, unsigned long *size);
 extern void * read_sha1_file(const unsigned char *sha1, char *type, unsigned long *size);
 extern int write_sha1_file(void *buf, unsigned long len, const char *type, unsigned char *return_sha1);

sha1_file.c

@@ -307,6 +307,19 @@ void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
 	return map;
 }
 
+int unpack_sha1_header(z_stream *stream, void *map, unsigned long mapsize, void *buffer, unsigned long size)
+{
+	/* Get the data stream */
+	memset(stream, 0, sizeof(*stream));
+	stream->next_in = map;
+	stream->avail_in = mapsize;
+	stream->next_out = buffer;
+	stream->avail_out = size;
+
+	inflateInit(stream);
+	return inflate(stream, 0);
+}
+
 void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, unsigned long *size)
 {
 	int ret, bytes;
@@ -314,18 +327,8 @@ void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, unsigned l
 	char buffer[8192];
 	unsigned char *buf;
 
-	/* Get the data stream */
-	memset(&stream, 0, sizeof(stream));
-	stream.next_in = map;
-	stream.avail_in = mapsize;
-	stream.next_out = (unsigned char *)buffer;
-	stream.avail_out = sizeof(buffer);
-
-	inflateInit(&stream);
-	ret = inflate(&stream, 0);
-	if (ret < Z_OK)
-		return NULL;
-	if (sscanf(buffer, "%10s %lu", type, size) != 2)
+	ret = unpack_sha1_header(&stream, map, mapsize, buffer, sizeof(buffer));
+	if (ret < Z_OK || sscanf(buffer, "%10s %lu", type, size) != 2)
 		return NULL;
 
 	bytes = strlen(buffer) + 1;