this.test.fullTitle() changes when using --watch

[fatso83]

Prerequisites

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Description

Adding --watch means this.test.fullTitle() prepends a space (' ') on consecutive runs.

Steps to Reproduce

git clone https://gist.github.com/fatso83/d6ef639119bc497cbc8db2e2a950e441 name-issue
cd name-issue/
npm i
npm test
sleep 2;
touch demo.test.js 

Will result in

1..1
ok 1 Outer TEST
# tests 1
# pass 1
# fail 0

  1..1
not ok 1  Outer TEST
  'Outer TEST' == ' Outer TEST'
  AssertionError [ERR_ASSERTION]: 'Outer TEST' == ' Outer TEST'
      at Context.<anonymous> (demo.test.js:7:16)
# tests 1
# pass 0
# fail 1

The actual test looks like this:

describe("Outer", function() {
    it("TEST", (function() {
        assert.equal('Outer TEST',this.test.fullTitle());
    }));
});

Expected behavior: [What you expect to happen]
No errors - the full title should stay the same

Actual behavior: [What actually happens]

A space gets prepended

Reproduces how often: [What percentage of the time does it reproduce?]
100%

Versions

6.0.2

$ mocha --version
6.0.2

$ node --version
v11.11.0

$ lsb_release  -a
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.2 LTS
Release:	18.04
Codename:	bionic

[fatso83]

I have debugged this now and found the cause of this. I'll submit a PR with the fix.