needs a way to specify default values for config options by path

Possibly the simplest way to fix this is to just give locations.conf lower priority than branch.conf? I think there may be another bug asking for that, or at least it would fix some other bugs about the masking.

- migration may be hard
- perhaps there are cases where you actually do want locations.conf to have higher priority?

> - perhaps there are cases where you actually do want locations.conf to have higher priority?

Given that locations.conf is documented as *overriding* values in branch.conf I assume there are people relying on this behaviour. I occasionally relies on it myself.

This bug is more about the misunderstandings about how sections are matched and ordered in locations.conf:

   [/dir]
   foo= 12
   [/dir/subdir]
   foo = 42

has exactly the same semantics as:

   [/dir/subdir]
   foo = 42
   [/dir]
   foo= 12

in locations.conf (see config.py and associated tests for other confusing use cases).