Change "mxcl/homebrew" refs to "Homebrew/homebrew"

Closes #24355.
Fix installer version comparison for 10.10+

Fixes #26018.
Closes #26040.
install: ENTER to RETURN

Since users will be on OS X, most will have a return key instead of an
enter key on their keyboard.

Closes #26042.
install: improve handling of shared groups.

Closes #26184.
Validate the output of xcrun more thoroughly

Noticed when installing Homebrew on a fresh install of 10.7 printed
this:

  -e:204: command not found: Error: /usr/bin/xcode-select returned unexpected error. --version
Just pass arguments
Use feature detection
Fix git version check
Invalidate sudo timestamp before exiting
*Quietly* invalidate the sudo timestamp
Installer should run warning-free
Tweak post-install text
Tweak post-install text further

Closes #28902.
Allow Xcode to satisfy the installer
No, really allow Xcode to satisfy the installer
Use <developer_dir>/usr/bin/git as a heuristic
Direct Linux users to Linuxbrew.

References #29813.
Be robust against existing remote.origin config

I will take this opportunity to opine and say that I don't think sharing
remote config settings across repositories makes sense. Remotes belong
to the repository, and the URL has to be configured anyway. It breaks
the assumption that you can run "git remote add origin" in a blank repo.
Repository templates are a much nicer way to address any perceived
duplication.

Nonetheless, git allows this to happen and there are a number of
misguided blogposts advocating it, so we should be robust against it.

Closes #31107.
Set the remote config manually
Use Current symlink instead of hardcode 1.8

The #! should point at `Current`, which is a symlink to the "system"
version of Ruby. This symlink exists by default on at least 10.6,
10.8, 10.9, and 10.10. In 10.10, Ruby 1.8 is not present, so the
install script fails to install brew on Yosemite beta (10.10).

Signed-off-by: Jack Nagel <jacknagel@gmail.com>