ident: fix type of string length parameter

ident: fix type of string length parameter

The last parameter in `split_ident_line()` is the length of the line
passed in by the caller. As such, most callers pass in either the result
of `strlen()`, `struct strbuf::len` or a pointer diff, all of which
are expected to be positive numbers. Regardless of that, the function
accepts a signed integer, which is somewhat confusing.

Fix the function signature to instead accept a `size_t`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>