read_sha1_file(): get rid of read_sha1_file_repl() madness

read_sha1_file(): get rid of read_sha1_file_repl() madness

Most callers want to silently get a replacement object, and they do not
care what the real name of the replacement object is.  Worse yet, no sane
interface to return the underlying object without replacement is provided.

Remove the function and make only the few callers that want the name of
the replacement object find it themselves.

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