ExternalProject: Use ${ref}^0 to avoid MSYS/Cygwin shell substitutions

In 5e941a545b (ExternalProject: Ensure git fetch if updating to hash we
don't have yet, 2021-05-09), the form ${ref}^{commit} was used to ensure
the ref was treated as a commit. When running under a MSYS2 or
Cygwin shell, the {commit} part can be translated by the shell, leaving it as
${ref}^commit, which git doesn't understand. Use the equivalent but
safer form ${ref}^0, since it won't be subject to the shell substitution
issue.

Fixes: #22299

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