Fix a usage error in a call to the object_file_name() function

In 82324ac1, the new static function exists_loose() called
object_file_name() and, in order to detect an error return,
tested for a negative value. This usage is incorrect, as
the error return is indicated by a positive return value.
(A successful call is indicated by a zero return value)

The only error return from object_file_name() relates to
insufficient buffer space and the return value gives the
required minimum buffer size (which will always be >0).

If the caller requires a dynamically allocated buffer,
this allows something like the following call sequence:

    size_t len = object_file_name(NULL, 0, db->object_dir, id);
    char *buf = git__malloc(len);
    if (!buf)
        error(...);
    object_file_name(buf, len, db->object_dir,id);
    ...

No current callers take advantage of this capability.

Fix up the call site and change the return type of the
function, from int to size_t, which more accurately
reflects the implementation.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>

src/odb.c

@@ -185,7 +185,7 @@ int git_obj_hash(git_oid *id, git_obj *obj)
 	return GIT_SUCCESS;
 }
 
-static int object_file_name(char *name, size_t n, char *dir, const git_oid *id)
+static size_t object_file_name(char *name, size_t n, char *dir, const git_oid *id)
 {
 	size_t len = strlen(dir);
 
@@ -860,7 +860,7 @@ static int exists_loose(git_odb *db, const git_oid *id)
 {
 	char file[GIT_PATH_MAX];
 
-	if (object_file_name(file, sizeof(file), db->objects_dir, id) < 0)
+	if (object_file_name(file, sizeof(file), db->objects_dir, id))
 		return 0;
 
 	if (gitfo_exists(file) < 0)