Remove .gitmodules if it was created in the same commit that introduced the submodule

If the `.gitmodule` file was created in the same commit that first mentions submodule we're currently merging, `.gitmodules`should be removed. The rationale: if the user didn't have it and Git created it for them, the user probably didn't care about the file at all, and would not have created it manually on this commit. (Generally we can't assume that the user didn't create something on a whim, but in this case I think we can be pretty sure.)