builtin-commit: fix --signoff

builtin-commit: fix --signoff

The Signed-off-by: line contained a spurious timestamp.  The reason was
a call to git_committer_info(1), which automatically added the
timestamp.

Instead, fmt_ident() was taught to interpret an empty string for the
date (as opposed to NULL, which still triggers the default behavior)
as "do not bother with the timestamp", and builtin-commit.c uses it.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>