Use keep(1) instead of add() when skipping an entry

Use keep(1) instead of add() when skipping an entry

Doing a keep call with a length of 1 will copy the current entry just
like the previous add was doing, but it avoids doing any validation
on the entry.  This is sane because the entry can be assumed to be
already valid, since its originating from the destination index.

Change-Id: I250d902fc98580444af1ba4b8fedceb654541451
Originally: http://thread.gmane.org/gmane.comp.version-control.git/128214/focus=128213
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>