one of the other git tools that git-subtree calls. Should
detect this situation and print the *real* problem.
- In fact, the prefix should *not* end with slash, and we
- should detect (and fix) it if it does. Otherwise the
- log message looks weird.
-
"pull --squash" should do fetch-synthesize-merge, but instead just
does "pull" directly, which doesn't work at all.