Detect path names which overflow the name length field in the index

Detect path names which overflow the name length field in the index

C Git allows a path name to be longer than 4095 bytes by storing 4095
into the path name length field within flags and then searching for a
null terminator at the end of the path name, instead of relying on the
length indicatior.  We cannot do this (easily) from an InputStream so
we are currently going to just abort with an exception if we find such
an extremely long path name.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>