add info on how to move additional branches

docs/development/howtos/vendor-a-repo.rst

@@ -101,7 +101,102 @@ Done!
 If giving is to cease to exist as a repo, move open issues to receiving's repo.
 That will not be detailed here.
 
+Moving issues
+-------------
+
+If `giving` is on Github, it seems Github is able to handle this just fine.
+
 Move other branches from giving to receiving
 --------------------------------------------
 
-TBD.
+Move PR
+-------
+
+If you want to move a PR to the argus-server repo (receiving) you first need to
+move the branch, then start a new PR with the same name at argus-server, then
+link back to any discussion in this repo from the new PR. Now you can close the
+PR in this repo.
+
+Prep code in giving
+-------------------
+
+In your local copy of giving (or a new clone) make a branch off main called rename::
+
+        git switch -c rename
+
+Move the code of main (just the code) to the paths that are correct for
+receiving. Feel free to also update import paths and template paths in this
+code. Commit the changes to the ``rename`` branch.
+
+If the code is going into a new subdirectory, make sure the parent directory is
+empty. If necessary ``git rm`` the ``__init__.py`` file or any others.
+
+Setup the remote in the giving repo
+-----------------------------------
+
+1. Add the receving repo as a remote::
+
+        git remote add receving url/to/reciving
+
+2. Fetch the branches on argus::
+
+        git fetch receiving
+
+3. Checkout master::
+
+        git switch master
+
+Move the actual branch
+----------------------
+
+First move your branch onto the ``rename`` branch.
+
+Do ``git mv old new`` or ``git rebase rename mybranch`` or use a graphical
+client to cherrypick one by one onto main, or copy the files to the correct
+place and add+commit them as new. It is enough to just move the files.
+Correcting import paths and file include paths can be done *after* the move, in
+the new repo, with a new commit.
+
+This way, you can deal with file renaming conflicts once, and content change
+conflicts once.
+
+Now you're ready for the move.
+
+1. Make a temporary branch name for the branch you want to move, at its head::
+
+        git switch mybranch
+        git switch -c fvgyhj
+
+2. If it's only a single commit you can cherrypick it. Move the real name to
+   the master then cherry-pick::
+
+        git branch -f mybranch master
+        git switch mybranch
+        git cherry-pick fvgyhj
+
+   If not, a rebase can do it for you. If you didn't do step 1 correctly there
+   will be more conflicts than necessary!
+
+   How to rebase (assumes ``mybranch`` is rebased on ``rename``)::
+
+        git rebase --onto master rename mybranch
+
+   You can now remove the temporary branch::
+
+        git branch -d fvgyhj
+
+3. Push the branch to the new remote::
+
+        git switch mybranch
+        git push receiving
+
+Make a new PR
+-------------
+
+1. Make the PR in receiving's repo and pull the branch in your local copy of
+   that repo.
+
+2. Do any internal changes to the actual code in the new repo if you didn't do
+   that as a part of the branch prep.
+
+Done!