subtree: don't have loose code outside of a function

subtree: don't have loose code outside of a function

Shove all of the loose code inside of a main() function.

This comes down to personal preference more than anything else.  A
preference that I've developed over years of maintaining large Bash
scripts, but still a mere personal preference.

In this specific case, it's also moving the `set -- -h`, the `git
rev-parse --parseopt`, and the `. git-sh-setup` to be closer to all
the rest of the argument parsing, which is a readability win on its
own, IMO.

"Ignore space change" is probably helpful when viewing this diff.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>