fmt: accept repeated arguments

[BenWiederhake]

Turns out, just a simple args_override_self(true) is precisely the correct behavior.

This is also correct with regard to -p, -w, -g; I checked (and found #6352, #6353, #6354 in the process of doing so).

This is work towards #5998.

Note that even though I added some basic tests, a huge chunk of the functionality is still untested, especially -c, -x/-X, -t, -m, -q, -T, and of course -g, which is broken (known issue).

[BenWiederhake]

Changes since last push: Forgot to commit the test data, whoops :D

[BenWiederhake]

Remaining failures are due to #6275 and #6333.

[BenWiederhake]

Changes since last push:

• rebased
• extracted common arg to common arg() (thanks @cakebaker!)
• use stdout_is_fixture as appropriate (thanks @cakebaker!)

[BenWiederhake]

Changes since last push:

• Now that at_and_ucmd is no longer needed, don't use it. (duh)

[cakebaker]

Thanks!