fast-export: rename --signed-tags='warn' to 'warn-verbatim'

fast-export: rename --signed-tags='warn' to 'warn-verbatim'

The --signed-tags= option takes one of five arguments specifying how to
handle signed tags during export.  Among these arguments, 'strip' is to
'warn-strip' as 'verbatim' is to 'warn' (the unmentioned argument is
'abort', which stops the fast-export process entirely).  That is,
signatures are either stripped or copied verbatim while exporting, with
or without a warning.

Match the pattern and rename 'warn' to 'warn-verbatim' to make it clear
that it instructs fast-export to copy signatures verbatim.

To maintain backwards compatibility, 'warn' is still recognized as
deprecated synonym of 'warn-verbatim'.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>