ExternalProject: note the default of `GIT_TAG` being `master`

Ben Boeckelrequested to merge
ben.boeckel/cmake:ExternalProject-default-git-tag into master

Reported on Discourse:

https://discourse.cmake.org/t/fetchcontent-makeavailable-invalid-reference-master/6386

Cc: @brad.king
Backport: release

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