Upgrade transcrypt to latest v2 upstream and add fallback v1 compatib…

…le version

The change updates the `transcrypt` script used by default for secret
file management to an amended version 2+ from upstream, to gain a
number of features and improvements I have been making there:

- add a merge driver to better handle merges of secret file changes,
  to automatically merge when possible and leave human-friendly
  change markers in conflicting files if not
- add pre-commit hook to prevent accidental commit of secret files as
  plaintext
- fix handling of files with special characters or spaces in the path
- unit tested!

This change also adds the `transcrypt-v1-compatible` script which is
a custom modified version of the latest `transcrypt` that is
compatible with the (faulty) salt generation technique used prior to
version 2. For existing projects where fully upgrading transcrypt is
too difficult or not worth it, copy `transcrypt-v1-compatible` into
the repo's `bin/` directory and name it `transcrypt` to gain all the
latest improvements while keeping compatibility with the old version
1 approach.

Scripts are taken from IC's fork https://github.com/ixc/transcrypt/
with `transcrypt` at commit bb2df81 and `transcrypt-v1-compatible`
at aec985b.