I have four separate projects. They have their own git repository. and the same name of branches for all projects.
/project/
/project/projA/
/project/proj
That solution is not so far from what you tried. This works only if your different projects have no common files (otherwise it can be difficult to solve conflicts)
# create a new repo:
git init all_projects
cd all_project
# to make it more easy to understand, let's create a new branch
git checkout -b main
# import 1st project
git remote add projectA http://projectA
git fetch --all --tags
git checkout masterA projectA/master
git rebase masterA main
# move the main branch to the current state
git branch main -f
# Now your branch main is at the same state as your A project
# import 2st project
git remote add projectB http://projectB
git fetch --all --tags
git checkout masterB projectB/master
git rebase masterB main
# move the main branch to the current state
git branch main -f
# Now your branch main contains all commits from projectA and projectB
# etc..
The result will be a repository with 1st all commits from project A, then all commits from project B, even if the project B has some commits dated before the last commit of project A, but this should not be a problem (and the history tree will be easier to read)
EDIT : Sorry I just notice this not solve your problem to get all remote branches. Maybe you can find a solution based on that question, with something like this:
for i in $(git branch -r |grep projectA|awk -F'/' '{print $2}'); do
git checkout $i projectA/$i
git rebase $i main
done
but this would make your tree more complex because all branches will starts from the main
commit ..