According to the documentation for git mergetool:

If no <file> names are specified, git mergetool will run the merge tool program on every file with merge conflicts.

We need support for this, to emulate the efficient conflict-resolution workflow we are used to from Plastic SCM, where Plastic's mergetool (which I recently contributed as an ExternalMergeTool in SourceGit) is also capable of "silently" resolving some conflicts automatically (using its --automatic flag).

I've created a PR for this feature, which was easy to implement.