git-gui: Automatically reopen any console closed by the user.

git-gui: Automatically reopen any console closed by the user.

If the user closes a console and we get more ouptut for it then we
will get a Tcl error in the readable event handle for the file channel.
Since this loses the actual output and is quite unfriendly to the end
user instead reopen any console which the user closed prior to the
additional output arriving.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>