ident.c: add split_ident_line() to parse formatted ident line

The commit formatting logic format_person_part() in pretty.c
implements the logic to split an author/committer ident line into
its parts, intermixed with logic to compute its output using these
piece it computes.

Separate the former out to a helper function split_ident_line() so
that other codepath can use the same logic, and rewrite the function
using the helper function.

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