Support external subcommands: rg, diff, git-show (etc.) (#1769)

* Support external subcommands: rg, git show, git log (etc.)

The possible command line now is:

  delta <delta-args> [SUBCMD <subcmd-args>]

If the entire command line fails to parse because SUBCMD is unknown,
then try (until the next arg fails) parsing <delta-args> only,
and then parse and call SUBCMD.., its output is piped into delta.
Other subcommands also take precedence over the diff/git-diff mode
(`delta a b`, where e.g. a=git and b=show), and any diff call gets
converted into an external subcommand first.

Available are:
  delta rg ..       => rg --json .. | delta
  delta a b ..      => git diff a b .. | delta
  delta git show .. => git <color-on> show .. | delta

and all other git-CMDS, of which
add -p, blame, checkout -p, diff, grep, log -p, reflog -p, and stash show -p
produce a diff.

Because --json is automatically added for `delta rg ..`, it avoids the
parsing ambiguities of and is easier to type than `rg .. | delta`.

The piping is not done by the shell, but delta, so the subcommands
are now child processes of delta.

* Set calling process directly because delta started it

This info then takes precedence over whatever
start_determining_calling_process_in_thread() finds or rather
doesn't find.
(The simple yet generous SeqCst is used on purpose for the atomic operations.)