Correctly skip over unrecognized optional dircache extensions

Correctly skip over unrecognized optional dircache extensions

We didn't skip the correct number of bytes when we skipped over an
unrecognized but optional dircache extension.  We missed skipping
the 8 byte header that makes up the extension's name and length.

We also didn't include the skipped extension's payload as part of
our index checksum, resuting in a checksum failure when the index
was done reading.  So ensure we always scan through a skipped
section and include it in the checksum computation.

Add a test case for a currently unsupported index extension, 'ZZZZ',
to verify we can still read the DirCache object even though we
don't know what 'ZZZZ' is supposed to mean.

Bug: 301287
Change-Id: I4bdde94576fffe826d0782483fd98cab1ea628fa
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>