Changes to root module are not detected correctly

[buddhike]

Which mbt command are you having troubles with?
all commands

What's the current behaviour?
When we have a module at the repository root (not a very common scenario), changes to the content in the root directory does not cause the root module to be treated as a modified.

To reproduce

mkdir test
cd test
git init 
echo "name: test" > .mbt.yml
git add -A
git commit -m "first"
git checkout -b "branch"
echo "hello" > foo
git add -A
git commit -m "second"
mbt describe pr --src branch --dst master
# returns an empty results set

What's the expected behaviour?
should display test module as modified

Environment
all environments