fix off-by-one error in split_ident_line

fix off-by-one error in split_ident_line

Commit 4b340cf split the logic to parse an ident line out of
pretty.c's format_person_part. But in doing so, it
accidentally introduced an off-by-one error that caused it
to think that single-character names were invalid.

This manifested itself as the "%an" format failing to show
anything at all for a single-character name.

Reported-by: Brian Turner <bturner@atlassian.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>