Improve reuse of sha1_file library within fast-import.

Improve reuse of sha1_file library within fast-import.

Now that the sha1_file.c library routines use the sliding mmap
routines to perform efficient access to portions of a packfile
I can remove that code from fast-import.c and just invoke it.
One benefit is we now have reloading support for any packfile which
uses OBJ_OFS_DELTA.  Another is we have significantly less code
to maintain.

This code reuse change *requires* that fast-import generate only
an OBJ_OFS_DELTA format packfile, as there is absolutely no index
available to perform OBJ_REF_DELTA lookup in while unpacking
an object.  This is probably reasonable to require as the delta
offsets result in smaller packfiles and are faster to unpack,
as no index searching is required.  Its also only a temporary
requirement as users could always repack without offsets before
making the import available to older versions of Git.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>