GitRequirement: use Utils.git_available?
GitRequirement: use Utils.git_available?

`which("git")` will return incorrect result for OS X without Xcode/CLT
installed, where `/usr/bin/git` is a stub.

Closes #46386.

Signed-off-by: Xu Cheng <9a05244150b861e40c78843b801eed71bca54eac@me.com>

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