Inspirated by I started to teach propellor about other ARM boards. (After having a clean build for a supported ARM board)

Using two directories, a ~/.propellor with my hosts and src/propellor with propellor source code.

In the code directory I modified src/Propellor/Property/Machine.hs, compiled with debuild -uc -us and installed with dpkg -i ../propellor*.deb.

Then using my hosts directory to get a WTF moment

$ propellor --spin
Auto-merging src/Propellor/Property/Machine.hs
Auto-merging propellor.cabal
Auto-merging debian/changelog
Auto-merging config.hs
CONFLICT (add/add): Merge conflict in config.hs
Automatic merge failed; fix conflicts and then commit the result.
propellor: Failed to run git ["merge","279b9267952b598914037983f74606d4f9c4ff6e","-s",
    "recursive","-Xtheirs","--quiet","-m","merging upstream version"
CallStack (from HasCallStack):
  error, called at src/Propellor/DotDir.hs:425:17 in main:Propellor.DotDir

What did connect both directories and why?

More important:

What directory setup and workflow to use to teach propellor about other ARM boards?