Fix --signoff in builtin-commit differently.

Fix --signoff in builtin-commit differently.

Introduce fmt_name() specifically meant for formatting the name and
email pair, to add signed-off-by value.  This reverts parts of
13208572fbe8838fd8835548d7502202d1f7b21d (builtin-commit: fix --signoff)
so that an empty datestamp string given to fmt_ident() by mistake will
error out as before.

Signed-off-by: Junio C Hamano <gitster@pobox.com>