diffuse cannot access history of a git submodule

A git submodule is a subdirectory of a local git repository that is itself a git repository.
The parent repository knows that this particular subdirectory is a separate repository with a separate history; the parent repository does not track the content of the submodule, but its history contains information about which revision is checked out in the submodule (and this information may be updated as needed).

When running diffuse inside a git submodule, it cannot access git history, so commands like the following do not work as expected:
diffuse -m
diffuse path/to/a/single/file
diffuse -r HEAD~1 -r HEAD path/to/file

I guess that the issue with git submodules is that inside the submodule, there is no .git directory, only a .git file that contains a line like the following:

gitdir: ../.git/modules/pyvallex

The real .git directory for the submodule can be found at the specified path.