[Config] CMake: Shallow clone when using the git-fetch mechanism

[fredroy]

No need to retrieve the full repository on fetching IMO

Example with BeamAdapter:

• full

git clone https://github.com/sofa-framework/BeamAdapter
...
Receiving objects: 100% (30388/30388), 44.09 MiB | 2.67 MiB/s, done.

• shallow

git clone https://github.com/sofa-framework/BeamAdapter -depth 1
...
Receiving objects: 100% (248/248), 10.09 MiB | 8.03 MiB/s, done.

Some caveats from CMake though : https://gitlab.kitware.com/cmake/cmake/-/issues/17770 , depending if GIT_TAG is a branch, a ref or... a tag (🤪)

NOTE: shallow clone does not work if GIT_TAG is hash SHA reference to a commit so I needed some cmake function to test if the argument is a hash.

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