Don't try to install from git if git is too old
Don't try to install from git if git is too old

This came up in tigerbrew, where the install failed for a user with
git 1.6.x: https://github.com/mistydemeo/tigerbrew/issues/9

TODO Given that this left the user with a chowned /usr/local but no
functional homebrew install, there should still be better error
handling here.

Fixes mistydemeo/tigerbrew#9.
Fixes #20945.
Closes #18666.

princeofdarkness76/homebrew,princeofdarkness76/homebrew,princeofdarkness76/homebrew,princeofdarkness76/homebrew