Backport: Add support for gcc as git submodule of another repository.

[Richard Earnshaw (lists)]

This is a straight backport of 
https://gcc.gnu.org/ml/gcc-patches/2018-04/msg01352.html.  Without this 
contrib/gcc_update fails if run from a worktree, while git pull works 
just fine.

Pushed to the gcc-8 branch.

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