commitmsg: rename "shiftTab()" signal to "leave()" for consistency

Signed-off-by: David Aguilar <davvid@gmail.com>
grep: replace the GrepLineEdit class with a QAction

Simpler, again ;-)

Signed-off-by: David Aguilar <davvid@gmail.com>
grep: allow moving between query and result using the arrow keys

The query input field already knew to focus the results when pressing
either enter, return, or the down-arrow key.  Teach the results field to
focus the input field when the up-arrow is pressed.

This makes the behavior consistent with the commit message editor.

Signed-off-by: David Aguilar <davvid@gmail.com>