object-file.c: use "enum" return type for unpack_loose_header()

In a preceding commit we changed and documented unpack_loose_header()
from its previous behavior of returning any negative value or zero, to
only -1 or 0.

Let's add an "enum unpack_loose_header_result" type and use it for
these return values, and have the compiler assert that we're
exhaustively covering all of them.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1307,7 +1307,10 @@ int git_open_cloexec(const char *name, int flags);
  * unpack_loose_header() initializes the data stream needed to unpack
  * a loose object header.
  *
- * Returns 0 on success. Returns negative values on error.
+ * Returns:
+ *
+ * - ULHR_OK on success
+ * - ULHR_BAD on error
  *
  * It will only parse up to MAX_HEADER_LEN bytes unless an optional
  * "hdrbuf" argument is non-NULL. This is intended for use with
@@ -1315,9 +1318,17 @@ int git_open_cloexec(const char *name, int flags);
  * reporting. The full header will be extracted to "hdrbuf" for use
  * with parse_loose_header().
  */
-int unpack_loose_header(git_zstream *stream, unsigned char *map,
-			unsigned long mapsize, void *buffer,
-			unsigned long bufsiz, struct strbuf *hdrbuf);
+enum unpack_loose_header_result {
+	ULHR_OK,
+	ULHR_BAD,
+};
+enum unpack_loose_header_result unpack_loose_header(git_zstream *stream,
+						    unsigned char *map,
+						    unsigned long mapsize,
+						    void *buffer,
+						    unsigned long bufsiz,
+						    struct strbuf *hdrbuf);
+
 struct object_info;
 int parse_loose_header(const char *hdr, struct object_info *oi,
 		       unsigned int flags);