devtool: add --force-patch-refresh to 'modify' and 'finish' commands

devtool: add --force-patch-refresh to 'modify' and 'finish' commands

This is very useful for updating patch context so that any fuzz is eliminated.
Simply issue:

devtool modify <recipe>
devtool finish --force-patch-refresh <recipe> <layer_path>

Without this flag, devtool will not deem the commits in the workspace
different to patches in the layer, even if the commits have different,
up-to-date context line in them.

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>