watch: implement --interval

[sylvestre]

      -n, --interval seconds
              Specify update interval.  The command will not allow quicker than 0.1 second inter-
              val,  in  which the smaller values are converted. Both '.' and ',' work for any lo-
              cales. The WATCH_INTERVAL environment can be used to persistently set a non-default
              interval (following the same rules and formatting).

[maxer137]

I am interested in creating an implementation for this.
Though, I am having some difficulty figuring out the code coverage locally before submitting a merge request.

Shall I create a merge request and then add the tests, or the other way around?
If I should add the tests first; Do you have a method of checking code coverage locally?

[sylvestre]

And both in the same pr

[cakebaker]

The main functionality has been implemented in the meantime. What remains is the handling of WATCH_INTERVAL.