PersonIdent: Document that name and email aren't trimmed

PersonIdent: Document that name and email aren't trimmed

This might be somewhat surprising behavior to users who might
naturally assume the following invariant:
  ident.equals(parseIdent(ident.toExternalString()))

This invariant does not hold since whitespace is only trimmed during
serialization. We don't want to mess with the strings during
initialization, as this is called during the highly-optimized commit
parsing codepath.

Change-Id: I081a603f0ac0e33167462244779b0ff3ad51e80c