lock_ref_sha1_basic does not remove empty directories on BSD

lock_ref_sha1_basic relies on errno beeing set to EISDIR by the
call to read() in resolve_ref() to detect directories.  But calling
read() on a directory under NetBSD returns EPERM, and even succeeds
for local filesystems on FreeBSD.

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -42,6 +42,12 @@ const char *resolve_ref(const char *path, unsigned char *sha1, int reading)
 			}
 		}
 
+		/* Is it a directory? */
+		if (S_ISDIR(st.st_mode)) {
+			errno = EISDIR;
+			return NULL;
+		}
+
 		/*
 		 * Anything else, just open it and try to use it as
 		 * a ref