Don't discard tagged nodes together with bookmarked ones
This fixes heptapod#96 (closed)
It was a bit light on my part to say that hgshas already
present in git_refs
were necessarily bookmarked: they
could also have been tagged.
We resist the temptation to build and pass this
hgshas_with_bookmark_git_ref
from get_exportable()
because we want ultimately this code to be in our own
extension that would rewrap hg-git
.