fast-export: Add a --tag-of-filtered-object option for newly dangling tags

fast-export: Add a --tag-of-filtered-object option for newly dangling tags

When providing a list of paths to limit what is exported, the object that
a tag points to can be filtered out entirely.  This new switch allows
the user to specify what should happen to the tag in such a case.  The
default action, 'abort' will exit with an error message.  With 'drop', the
tag will simply be omitted from the output.  With 'rewrite', if the object
tagged was a commit, the tag will be modified to tag an alternate commit.
The alternate commit is determined by treating the original commit as the
"parent" of the tag and then using the parent rewriting algorithm of the
revision traversal machinery (related to the "--parents" option of "git
rev-list")

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>