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.
Curl flags: <= 10.5

So I can be lazy and not change this downstream
Don't bother checking git version if none found
Handle failing git regex.
Fix "Homeprew" typo
Clean whitespace.
Tweak casing.
Install CLT on 10.9.
Stop using floats for version comparison
Improve Mavericks CLT installation usability.

* Install before Git is used.
* Warn about the GUI popup.
* Block on user input until installation finishes.
Update CLT link.
Change "mxcl/homebrew" refs to "Homebrew/homebrew"

Closes #24355.