Alire & git issue on macOS with SDK 15

[Simon Wright]

At the point where we go into ~/emacs.d/elpa/ada-mode-8.1.0 and call build.sh, 
the Alire path calls "alr get emacs_ada_mode~8.1.0". If you’re running macOS 14 
(Sonoma) with an updated Xcode or Command Line Tools, you may well see 
something like

Cloning into '/Users/simon/.emacs.d/elpa/ada-mode-8.1.0/alr-tgob.tmp'...
remote: Counting objects: 48758, done.
remote: Compressing objects: 100% (10336/10336), done.
remote: Total 48758 (delta 38298), reused 48758 (delta 38298)
Receiving objects: 100% (48758/48758), 240.39 MiB | 3.90 MiB/s, done.
Resolving deltas: 100% (38298/38298), done.
warning: remote HEAD refers to nonexistent ref, unable to checkout

This is because the repository at savannah.nongnu.org 
<http://savannah.nongnu.org/> doesn’t have a default branch, and the git that 
comes with the v15 SDK is 2.39.3 (it says "(Apple Git-146)" - I don’t know 
whether it’s been customised).

If you use Homebrew, "brew install git" gets you version 2.44, which doesn’t 
have this problem.