gowork-snapshot: Fix it for submodules

In the presence of Git submodules, the .git of a submodule is not a directory but a regular file with content like

gitdir: ../../.git/modules/vendor/libgit2

As -C path instructs git to cd into path this fails for submodule case with error like:

github.com/libgit2/git2go/vendor/libgit2        fatal: cannot change to './github.com/libgit2/git2go/vendor/libgit2/.git': Это не каталог

Fix it by changing -C to --git-dir which instructs Git to consider provided path as only location for git database, not to cd into it.

/cc @alain.takoudjou, @jerome