fast-export: check for unsupported signing modes earlier

fast-export: check for unsupported signing modes earlier

The '--signed-{commits,tags}' options for git-fast-export(1) support
only a subset of the modes accepted by git-fast-import(1). Unsupported
modes such as 'strip-if-invalid' and 'sign-if-invalid' are accepted
during option parsing, but cause the command to die later when a signed
object is encountered.

Instead, reject unsupported signing modes immediately after parsing the
option. This treats them the same as other unknown modes and avoids
deferring the error until object processing. This also removes
duplicated checks in commit/tag handling code.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>