Fix SHA1 checksum generation on little-endian machines.

git 2.13.0 broke SHA1 checksumming on big-endian machines. They fixed
it in 2.13.1 by breaking it on little-endian machines. This patch,
obtained from upstream, fixes SHA1 checksumming on little-endian
machines again; I don't have any BE machines but hopefully it still
works on those too.

Submitted by: Dan McGregor
Obtained from: https://github.com/git/git/commit/76339527ccf7946751174663f9faa686f3f55335
Approved by: just-fix-it