transport-helper: pass --signed-tags=warn-strip to fast-export

transport-helper: pass --signed-tags=warn-strip to fast-export

Currently, attempting to push a signed tag to a remote helper which uses
fast-export results in the remote helper failing because the default
fast-export action for signed tags is "abort".  This is not helpful for
users because there is no way to pass additional arguments to
fast-export here, either from the remote helper or from the command
line.

In general, the signature will be invalidated by whatever transformation
a remote helper performs on a tag to push it to a repository in a
different format so the correct behaviour is to strip the tag.  Doing
this silently may surprise people, so use "warn-strip" to issue a
warning when a signed tag is encountered.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>