Merge branch 'cb/avoid-colliding-with-netbsd-hmac'

Merge branch 'cb/avoid-colliding-with-netbsd-hmac'

The <stdlib.h> header on NetBSD brings in its own definition of
hmac() function (eek), which conflicts with our own and unrelated
function with the same name.  Our function has been renamed to work
around the issue.

* cb/avoid-colliding-with-netbsd-hmac:
  builtin/receive-pack: avoid generic function name hmac()