Fix parsing of multiple authors in PersonIdent.

PersonIdent should be parsable for an invalid commit which
contains multiple authors, like "A <a@a.org>, B <b@b.org>".
PersonIdent(String) constructor now delegates to
RawParseUtils.parsePersonIdent().

Change-Id: Ie9798d36d9ecfcc0094ca795f5a44b003136eaf7