Git v2.13.1 SHA1 very broken

[Adam Dinwoodie <adam@xxxxxxxxxxxxx>]

I'm trying to compile Git v2.13.1 to release for Cygwin, but it appears
a010391 ("sha1dc: update from upstream", 2017-05-20) is breaking a very
significant number of test cases in both 32-bit and 64-bit Cygwin
builds.

The first failure is t0000.46 "validate object ID of a known tree"; output with
-x and -v is below, although it's not very interesting:

    expecting success:
	    test "$tree" = 7bb943559a305bdd6bdee2cef6e5df2413c3d30a

    ++ test ceb282701536fe61bea01075664405caa7d6343f = 7bb943559a305bdd6bdee2cef6e5df2413c3d30a
    + test_eval_ret_=1
    + want_trace
    + test t = t
    + test t = t
    + set +x
    error: last command exited with $?=1
    not ok 46 - validate object ID of a known tree
    #
    #               test "$tree" = 7bb943559a305bdd6bdee2cef6e5df2413c3d30a
    #

I have no idea where to even begin debugging this, but I'm happy to take
pointers / try things out on my box.

Cheers,

Adam