git fetch --dry-run can modify the repo

Interesting bug we came across -- or at least I think it's a bug. In
any git repo that is not a shallow clone:

$ cat .git/shallow # "No such file or directory"
$ git fetch --depth=1 --dry-run
$ cat .git/shallow # Prints a git sha

I would think no command should make any changes to the local repo if
--dry-run is passed.

Thanks!

Tyler