Reduce Restic Progress FPS

[cimnine]

By defining the environment variable RESTIC_PROGRESS_FPS it is possible to reduce restic's output.

The reason to introduce this change is that we suspect that the buffer of the scanner is eventually filling up. This PR does not fix that permanently, but it reduces the output of restic dramatically, which should at least should delay the occurence of such errors until a more permament is found.
Also, wrestic does not need the progress to be reported every 1/60th of a second.

The behavior now – as implemented – is that if RESTIC_PROGRESS_FPS is defined in the environment variables that are passed to wrestic, the given value will be used. Otherwise, wrestic will set the value to about one progress report line per minute (i.e. 1.0f/60.0f per Second).

Originally discussed in #79 (comment).

[Kidswiss]

LGTM

[cimnine]

@Kidswiss can you have another look.

I've changed this function from:

func (c *Command) setResticProgressFPSIfNotDefined(givenEnv []string) []string {
	newEnv := make([]string, len(givenEnv))
	copy(newEnv, givenEnv)

	for i := range newEnv {
		if strings.HasPrefix("RESTIC_PROGRESS_FPS=", newEnv[i]) {
			return newEnv
		}
	}

	const frequency = 1.0 / 60.0
	c.cmdLogger.Info("Defining RESTIC_PROGRESS_FPS", "frequency", frequency)
	return append(newEnv, fmt.Sprintf("RESTIC_PROGRESS_FPS=%f", frequency))
}

to

func (c *Command) setResticProgressFPSIfNotDefined(givenEnv []string) []string {
	for _, envVar := range givenEnv {
		if strings.HasPrefix("RESTIC_PROGRESS_FPS=", envVar) {
			return givenEnv
		}
	}

	const frequency = 1.0 / 60.0
	c.cmdLogger.Info("Defining RESTIC_PROGRESS_FPS", "frequency", frequency)
	return append(givenEnv, fmt.Sprintf("RESTIC_PROGRESS_FPS=%f", frequency))
}

I.e. I removed the copy because I don't think it is needed: When we don't change the array, it's definitely not needed. And when we append, we'll get a new reference anyway.

[Kidswiss]

You're right, it's not needed to create a copy. LGTM