tools: moveconfig: Add a new --git-ref option

tools: moveconfig: Add a new --git-ref option

This option allows the 'make autoconf.mk' step to run against a former
repo state, while the savedefconfig step runs against the current repo
state. This is convenient for the case where something in the Kconfig
has changed such that the defconfig is no longer complete with the new
Kconfigs. This feature allows the .config to be built assuming those old
Kconfigs, but then savedefconfig based on the new state of the Kconfigs.

If in doubt, always specify this switch. It will always do the right
thing even if not required, but if it was required and you don't use it,
the moved configs will be incorrect. When not using this switch, you
must very carefully evaluate that all moved configs are correct.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>