Fix infinite loop when parsing invalid flags for apps with short option handling

[rliebz]

The old implementation of iterative parsing assumed that the error messages returned from the flag library would come in like this:

$ my-app -invalid
flag provided but not defined: -invalid
$ my-app --invalid
flag provided but not defined: --invalid

It turns out the error message for both is flag provided but not defined: -invalid.

This faulty assumption meant the iterative flag-parsing logic used for short-option handling could get into an infinite loop, where the same arguments would be parsed each loop, producing the same error each time with no exit condition.

To reproduce:

package main

import (
	"log"
	"os"

	"github.com/urfave/cli"
)

func main() {
	app := cli.NewApp()
	app.UseShortOptionHandling = true
	log.Fatal(app.Run(os.Args))
}

The following will hang forever, as will any flag prefixed by --:

$ go run main.go --invalid

The new test cases now cover that scenario.

I also cleaned up the arg rebuilding logic a bit to avoid unnecessary appends and loop iterations, and I added a condition to guarantee that an infinite loop cannot occur for any reason in the future, even though that code path should theoretically be unreachable as it stands.

[codecov]

Codecov Report

Merging #911 into master will decrease coverage by 0.07%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
- Coverage   71.45%   71.38%   -0.08%     
==========================================
  Files          30       30              
  Lines        2393     2394       +1     
==========================================
- Hits         1710     1709       -1     
- Misses        577      578       +1     
- Partials      106      107       +1

Impacted Files	Coverage Δ
parse.go	89.74% <77.77%> (-5%)	⬇️

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[coilysiren]

✨ thanks! ✨

[saschagrunert]

LGTM, thanks!