Panic when diff between actual and expected output adds a new line at the beginning of the output

[livioribeiro]

• assert_cli version: 0.6.1
• Rust version: 1.26.1
• OS and version: Ubuntu 18.04

When comparing the actual output with the expected output, if a new line is added at the beginning of the output, the following panic happens:

failures:

---- test stdout ----
	thread 'test' panicked at 'attempt to subtract with overflow', ~/.cargo/registry/src/github.1485827954.workers.dev-1ecc6299db9ec823/assert_cli-0.6.1/src/diff.rs:18:29

The problem happens at line:

assert_cli/src/diff.rs

Line 18 in 973ebcf

match diffs[i - 1] {

If i is 0, meaning that the diff is an addition to the first line, the panic happens.

For example, consider the code:

const EXPECTED: &str = "Line 1\nLine 2\nLine 3";
Assert::main_binary()
        .with_args(&args)
        .succeeds()
        .and().stdout().is(EXPECTED)

If the output is Line 0\nLine 1\nLine 2\n Line 3 (Line 0 added at the beginning) , the panic happens.