Merge branch 'export-subst'

Merge branch 'export-subst'

I wasn't able to completely use export-subst in .gitattributes to create
the contents of src/common/git_hash.h.  The %D substitution will return
the git branch followed a few lines later by the git tag, but only if
you're using it by way of git-log.  If you're using export-subst, you'll
only get the first line and there's no way to tell git to remove those
newlines.  So, I'm retaining the sed business on the Makefile.

See https://stackoverflow.com/questions/54525888/ for the question I asked
on the matter.