vcs pull does auto merge using `ort` strategy which alters the commits history

[andry81]

Currently git pull does that.

> vcs pull ...
...
Auto-merging ...
Merge made by the 'ort' strategy.

This leads to an auto merge of an external repository which alters the commits history.

Steps to repro:

1. Create an external repo Y
2. Create a commit with file A
3. Push Y
4. Create main repo X
5. Import the external repo Y (vcs import < ...)
6. Add file B to external repo Y
7. Commit with amend
8. Push external repo Y using force with lease
9. Pull a directory with the external repo Y (vcs pull ...)
10. Git does ort auto merge of the external repo Y, repo Y history is desynchronized now

I've catched that initially in the Dirk Thomas repo fork, so don't known is that applies to yours or not.