WIP: Install transcrypt script into repo

[jmurty]

When initializing transcrypt, copy the whole script file into the repository and make it runnable with the git transcrypt alias.

This approach gives a number of benefits:

• You can always run transcrypt within an initialized repo, even if the wider system no longer has it installed
• When you run git transcrypt within a repo you are guaranteed to be running the appropriate version of the script, even if the system's version has changed
• The clean, smudge, textconv, and merge scripts become trivial, just invocations of the main script
• The old contents of the clean, smudge, textconv, and merge scripts become part of the main script, and thus check-able and lint-able

[jmurty]

Hi @elasticdog what do you think about this approach of copying the entire transcrypt file into a repo on initialisation?

I was inspired by an idea from someone else's suggestion in an issue or PR from ages ago, I can't find the reference right now to give credit. But I think it's a good idea.

[jmurty]

Could we avoid user confusion by copying the script into the crypt dir to use internally, without creating the Git alias or telling the user?

[jmurty]

I have removed the git transcrypt alias, so the fact the original transcrypt script is copied into the repo's crypt/ directory is now hidden from the user: and internal implementation detail they don't need to know or care about.