Only replace absolute -> relative paths for stack lines

[Turbo87]

This PR resolves #2126.

[Turbo87]

@danielstjules any comments on this?

[danielstjules]

It looks good, but can you post the output of the code snippet from #2126 (comment) with your branch? Will take a look later tonight.

[Turbo87]

@danielstjules it will change to

  1) foo bar:
     Error: this is a test in /Users/tbn/Code/tmp/mocha-stack-bug/foo/bar/test.js and it fails
       at Context.<anonymous> (test/test.js:5:11)

in other words: the paths in the stacktrace are still simplified, but any paths in the original error message remain the same

[Turbo87]

@danielstjules ping

[danielstjules]

Sorry for the delay! Confirmed that the following works as described after the PR:

describe('foo', function() {
  it('bar', function() {
    throw new Error('this is a test in ' + process.cwd() + '/foo/bar/test.js and it fails');
  });
});

----------------
Before PR
----------------
  1) foo bar:
     this is a test in /Users/danielstjules/git/mocha/foo/bar/test.js and it fails
  Error: this is a test in foo/bar/test.js and it fails

----------------
After PR
----------------
  1) foo bar:
     Error: this is a test in /Users/danielstjules/git/mocha/foo/bar/test.js and it fails

Edit: Fixed output

[danielstjules]

What's the capture group for? Could this be simplified, e.g.

if (line.match(/:\d+:\d+$/)) {

[Turbo87]

that's actually not a capture group, those are escaped parenthesis. your suggestion would only work for stack lines without parenthesis around the file+position info, but according to the test/utils.js both seem to exist.

[danielstjules]

My bad, didn't notice the escape characters :)

[danielstjules]

Other than my comment on the regexp, looks good :)