Allow to specify base repository for modify

[mdellweg]

fixes #618

[ggainey]

pulp-cli-deb and pulp-cli-ostree plugins will need to add this as well, yes?

[mdellweg]

They should. But it's not like they will be broken immediately by this.
Until adoption, repositories need to be specified capped and gowned.

[patchback]

Backport to 0.30: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply ddd7fbb on top of patchback/backports/0.30/ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149/pr-1132

Backporting merged PR #1132 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulp-cli.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/0.30/ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149/pr-1132 upstream/0.30

4. Now, cherry-pick PR Allow to specify base repository for modify #1132 contents into that branch:

   $ git cherry-pick -x ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149

   If it'll yell at you with something like fatal: Commit ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Allow to specify base repository for modify #1132 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/0.30/ddd7fbb9430f7672ce44dbaac66e6c89a9ae9149/pr-1132

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
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