Because of this messing with the user's path:
https://github.com/Homebrew/brew/blob/
efc02899c851c62c9ce0d15dea9a231575d7d774/bin/brew#L68
brew uses /usr/bin/git over brewed git, even when the former is
problematically old.
There may also be other reasons a user prefers to use brewed git.
There was already a HOMEBREW_FORCE_BREWED_CURL option and a
HOMEBREW_SYSTEM_CURL_TOO_OLD check to set it. This mostly copies those
to implement HOMEBREW_FORCE_BREWED_GIT & HOMEBREW_SYSTEM_GIT_TOO_OLD.
See also: https://github.com/Linuxbrew/brew/issues/736
9fd5a92ff6