List all commits that modified the .gitmodules file ( this happens for example when you add/remove submodule)

git log --pretty=tformat:"%H" --follow -- ".gitmodules"

Get the contents of .gitmodules file at a specific commit

git show ${commitId}:.gitmodules

Print contents of .gitmodules file for each commit that modified .gitmodules

git log --pretty=tformat:"%H" --all --follow -- ".gitmodules" | \
(while read hash; do \
   git show $hash:.gitmodules > {BASE_DIRECTORY}/$hash 2> {BASE_DIRECTORY}/$hash.error; \
done \
);

In the above command --all is used to include all refs and not only current HEAD. Also, BASE_DIRECTORY should exist before running the command otherwise the command will fail. For each commit that modified .gitmodules file, two files will be created under BASE_DIRECTORY. One file will contain the contents of the .gitmodules file at that commit (empty if .gitmodules was empty at that commit). Other file will contain error that (if) happend while trying to read the contents of .gitmodules file at that commit.