git-gui: Avoid using the <<Copy>> binding as a menu accelerator on win32

git-gui: Avoid using the <<Copy>> binding as a menu accelerator on win32

On Windows the Control-C binding is used to copy and is mapped to the Tk
virtual event <<Copy>>. In the initial git-gui dialog this is also bound
as an accelerator for the Clone menu item. The effect is that both bindings
run, copying the text but resetting the clone page or switching to the clone
page when the user tries to copy text from one of the entry fields.
This patch avoids this by using Control-L instead for Windows only.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>