git_sha1_gen.py: add submodule .git directory When submodules are enabled, .git is a git link instead of a directory. We should find the new git directory location and then find HEAD revision there. Bug: b/253329946 Change-Id: I0ec69df186e0f0d106689f78792854db5858d31d Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/818669 Reviewed-by: Craig Stout <cstout@google.com>