close_sha1_file(): make it easier to diagnose errors

A bug report with "unable to write sha1 file" made us realize that we do
not have enough information to guess why close() is failing.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2241,7 +2241,7 @@ static void close_sha1_file(int fd)
 		fsync_or_die(fd, "sha1 file");
 	fchmod(fd, 0444);
 	if (close(fd) != 0)
-		die("unable to write sha1 file");
+		die("error when closing sha1 file (%s)", strerror(errno));
 }
 
 /* Size of directory component, including the ending '/' */