I'm not entirely sure, but `:` means "true" in bash, and if I omit it, something like this happens:

  $ git config alias.foo '! echo "$1";'
  $ git foo bar
  bar
  echo "$1";: bar: command not found

Whereas if I end with `; :` then it works as expected:

  $ git config alias.foo '! echo "$1"; :'
  $ git foo bar
  bar

It seems to execute the last argument (`bar`) as a command without the `:` at the end, and I don't have a `bar` command on my PATH so it angrily fails with exit code 127. If it instead executes `:` however, that will make it happily exit with 0.

It seems to be a Git alias quirk, but I may be incorrect here.