[CMake]: only warn when the working directory in a git worktree

[heitbaum]

• fixes #7559

fix the warning when the working directory is not a git worktree.

CMake Warning at CMake/Version.cmake:34 (message):
  git is not found
Call Stack (most recent call first):
  CMakeLists.txt:19 (include)

Signed-off-by: Rudi Heitbaum [email protected]

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[heitbaum]

@dbaileychess could you please review and let me know if any updates are required. Thanks

[dbaileychess]

@heitbaum I did, did you see my line comment?