ExternalProject: Ensure git fetch if updating to hash we don't have yet

In ac6a4d4884 (ExternalProject: Improve robustness of update step,
2020-10-17), the method used to check whether we already have a
commit or not was changed from using git rev-list to git rev-parse.
The new logic assumed rev-parse would output nothing if given a commit
hash it didn't know about, but it simply prints the hash again without
raising an error in this scenario. Amend that logic by adding ^{commit} to
the ref to ensure we do get an error if that ref is not currently known.

Fixes: #22166

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