Correct PersonIdent hashCode() and equals() to ignore milliseconds

Git doesn't store millisecond accuracy in person identity lines,
so a line that we create in Java and round-trip through a Git object
wouldn't compare as being equal.  Truncate to seconds when comparing
values to ensure the same identity is equal.

Change-Id: Ie4ebde64061f52c612714e89ad34de8ac2694b07
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>