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>

ident.h

@@ -35,7 +35,7 @@ void reset_ident_date(void);
  * Signals an success with 0, but time part of the result may be NULL
  * if the input lacks timestamp and zone
  */
-int split_ident_line(struct ident_split *, const char *, int);
+int split_ident_line(struct ident_split *, const char *, size_t);
 
 /*
  * Given a commit or tag object buffer and the commit or tag headers, replaces