hg-fast-export.sh: handle gc'd marks

hg-fast-export.sh: handle gc'd marks

If there are any "unnamed" heads in the source repository, those
revisions will be imported successfully but not attached to any
ref and the next time gc is run they will be removed.

When git fast-import attempts to read the --import-marks file it
will complain (and die) when it gets to the mark that refers to the
now-deleted revision.

Avoid this issue by removing any no-longer-existing marks from the
input marks file.

This is never an issue for a full import, only for an incremental
import.

This change is only a partial solution for incremental imports.  It
works fine as long as no future import attempts to use the gc'd
"unnamed" head as a parent or something to be tagged.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>