fsck: do not give up too early in fsck_dir()

fsck: do not give up too early in fsck_dir()

When there is a random garbage file whose name happens to be 38-byte
long in a .git/objects/??/ directory, the loop terminated prematurely
without marking all the other files that it hasn't checked in the
readdir() loop.

Treat such a file just like any other garbage file, and do not break out
of the readdir() loop.

While at it, replace repeated sprintf() calls to a single one outside the
loop.

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