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

In ac6a4d48 (ExternalProject: Improve robustness of update step, 2020-10-17) from !5389 (merged), 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 (closed)
Backport: release