Re: Fix mk/fetch/github.mk

[Greg Troxel <gdt%lexort.com@localhost>]

I really don't like magic that looks at the "tag" and decides if it is a
hash or a tag.  AFAIK it is not forbidden to use a sha-1 as a tag, even
though that's asking for trouble.

I wonder if we should use GITHUB_HASH instead of TAG when the value
is...  actually a commit hash instead of a tag, and then have
GITHUB_TYPE default to hash when GITHUB_HASH is defined.

That results in more changes, but it leads to IMHO much cleaner
semantics.