read_sha1_file(): allow selective bypassing of replacement mechanism

read_sha1_file(): allow selective bypassing of replacement mechanism

The way "object replacement" mechanism was tucked to the read_sha1_file()
interface was suboptimal in a couple of ways:

 - Callers that want it to die with useful diagnosis upon seeing a corrupt
   object does not have a way to say that they do not want any object
   replacement.

 - Callers who do not want it to die but want to handle the errors
   themselves are told to arrange to call read_object(), but the function
   does not use the replacement mechanism, and also it is a file scope
   static function that not many callers can call to begin with.

This adds a read_sha1_file_extended() that takes a set of flags; the
callers of read_sha1_file() passes a flag READ_SHA1_FILE_REPLACE to ask
for object replacement mechanism to kick in.

Later, we could add another flag bit to tell the function to return an
error instead of dying and then remove the misguided "call read_object()
yourself".

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