Remove gitIgnoreTimestamp from abstract iterator API

Remove gitIgnoreTimestamp from abstract iterator API

This never should have been exposed on the top of the
AbstractTreeIterator type hierarchy.  There is no concept of a
timestamp in a canonical tree read from the object database, and
the time in the DirCache isn't what we want here either.

Actually all that we need is to find the files whose names are
".gitignore" and are below the root directory.  We can accomplish
that with a suffix filter, and process them immediately.

Change-Id: Ib09cbf81a9e038452ce491385c65498312e2916b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Charley Wang <chwang@redhat.com>
CC: Chris Aniszczyk <caniszczyk@gmail.com>
CC: Stefan Lay <stefan.lay@sap.com>
CC: Matthias Sohn <matthias.sohn@sap.com>