git-gui: use a tristate to control the case mode in the searchbar

The config is now called gui.search.case and can have the three values:
no/yes/smart. yes is the default.

It also resets the case detection in smart mode, when the entry field was
cleared by the use.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>