How to cherry-pick changes in gnulib in the future

[Markus Mützel]

I updated the `HACKING.md` file to take the deleted gnulib subrepository into 
account:
https://hg.savannah.gnu.org/hgweb/octave/rev/ecb8ffc51cfa

In the earlier version I read that the gnulib subrepository was used in the 
past to cherry-pick necessary changes when we didn't want to do a full update 
for a point release.
Is there still a process how this could be done now that we use the `bootstrap` 
script to `git update` to a set revision?

Markus