kmilligan

First, make repoA contain only the files from the subdirectory you care about. To do this, we're going to first create a new working branch, and then use the filter-branch function of git.

cd ~/repoA
git branch -b informative_branch_name
git filter-branch --subdirectory-filter branch/you/want/with/trailing/slash/ -- --all

If you ls, you'll see that the repo contains only the files from the desired subdirectory. It would be a poor decision to push at this time.

Next, visit repoB, and add repoA as a remote. After that, check out the branch you just created (informative_branch_name)