Attempt to delay prepare_alt_odb during get_sha1

Attempt to delay prepare_alt_odb during get_sha1

Not every input value passed to get_sha1 is an abbreviated SHA-1.
Its actually quite common for refs to be passed and for those
refs to resolve to full SHA-1s, in which case we may not need to
initialize the alternate object database list in this process.

I'm relocating the call to prepare_alt_odb closer to the code
that actually needs it to maintain the fix first introduced by
Junio in 99a19b43 (to avoid ambiguous SHA-1 abbreviations from
being accepted).  This allows us to avoid the alt_odb list setup
if we won't actually need it.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>