ExternalProject: Only add git config setting with git 1.7.7 or later

1cb65e680d (ExternalProject: Prevent the noisy detached head
messages on checkout, 2021-01-17) unconditionally added the advice.detachedHead
git config setting, but it requires git 1.7.7 or later. Since it isn't fatal to not
have it, just noisier, only add it when it is supported.

Fixes: #22206