[PATCH] gitk: make autoselect optional

[PATCH] gitk: make autoselect optional

Whenever a commit is selected in the graph pane, its SHA1 is
automatically put into the selection buffer for cut and paste.
However, some users may find this behavior annoying since it can
overwrite something they actually wanted to keep in the buffer.

This makes the behavior optional under the name "Auto-select SHA1",
but continues to default to "on".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>