Treat empty '/' as "find next"

Treat empty '/' as "find next"

When the user requests a forward or backwards search, if
they provide no search string then we just abort the search.
However, many other programs (such as less and vi) treat
this as "repeat the last search", so users may have their
fingers trained to do this rather than use the explicit
"find next" feature.

This patch causes tig, if there was a previous search, to
find the next (or previous, if "?" was used) match in such a
case.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>