emacs: Fix search tab completion in terminals

In X, Emacs distinguishes the tab key, which produces a 'tab event;
from C-i, which produces a ?\t event.  However, in a terminal, these
are indistinguishable and only produce a ?\t event.  In order to
simplify things, Emacs automatically translates from 'tab to ?\t (see
"Function key translations" in M-x describe-bindings), so functions
only need to be bound to ?\t to work in all situations.

Previously, the search tab completion code usedq (kbd "<tab>"), which
produced the event sequence [tab], which only matched the 'tab event
and hence only worked in X.  This patch changes it to (kbd "TAB"),
which matches the general ?\t event and works in all situations.