checkkconfigsymbols.py: support git's "^" syntax

Support git's "^" syntax for diffing two commits, for instance via
"--diff HEAD^^^..HEAD".

Signed-off-by: Michael Ellermann <mpe@ellerman.id.au>
Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>