Copy vendor component from previous debian version.

For example, when updating from 13.6.5 to 13.7.0,
cp ../gitaly_13.6.5+dfsg.orig-vendor.tar.xz ../gitaly_13.7.0+dfsg.orig-vendor.tar.xz

To update the vendored modules to new upstream version.
1. Extract the vendor tarball 
	tar -xvf gitaly_15.8.4+dfsg.orig-vendor.tar.xz
2. Navigate to the required directory inside gitaly-vendor/
3. Replace the module with new upstream version either by manually downloading the new tarball,extract and replace or use go get
4. compress the gitaly-vendor/ with the updated name to the tarball by appending `1` to dfsg 
	tar -cvJf gitaly_15.8.4+dfsg1.orig-vendor.tar.xz gitaly-vendor/ 
