Proposal: Add --patches-directory option for epm update

[psibi]

$HOME/.epm/patches is a git repository tracking the eta patches for
hackage ecosystem.

If the --patches-directory option is given, after the git pull also
copy all the files in the patches directory to the git repository.
One cases that needs to be handled:

• git pull fails because of conflict (because you have already given
  the --patches-directory option). In that case - unstage the uncommited
  files and pull again.

Pros:

• Manually copying the patches to git directory $HOME/.epm/patches may
  cause further epm update to fail because of conflict. This
  proposal if implemented, can solve it in a much more intuitive
  way. (Practically, that can be solved without implementing this
  option and just handling the edge case in the update command.)

Cons:

• Leads to more code and more maintenance. Is it worth implementing ?

[rahulmutt]

I think this would assist package testing quite well, so I'm supportive of this. The only thing I'd like to add is that there should be a warning to the user that their old patches will get overwritten upon updating. Any hidden behaviour should be made explicit so the user isn't taken aback.