Fix edge cases / Improve mem. usage

[SpacePossum]

• Remove reference mismatch on finding same ending of array.
• Do not crash on undefined var. on PHP7
• More tests

Tagged as WIP as some cases still are missing test coverage, but I would like Travis to report on the work so far.
After this enough groundwork is done to start adding line numbers.

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 8.43%.
The diff coverage is 90.62%.

@@             Coverage Diff              @@
##             master      #40      +/-   ##
============================================
+ Coverage     86.41%   94.84%   +8.43%     
- Complexity      107      108       +1     
============================================
  Files             7        7              
  Lines           287      291       +4     
============================================
+ Hits            248      276      +28     
+ Misses           39       15      -24

Impacted Files	Coverage Δ	Complexity Δ
src/Parser.php	100% <100%> (+8.51%)	19 <0> (ø)	⬇️
src/Differ.php	89.05% <90.32%> (+4.72%)	54 <10> (+1)	⬆️
src/Chunk.php	100% <0%> (+11.11%)	7% <0%> (ø)	⬇️
src/Diff.php	100% <0%> (+100%)	5% <0%> (ø)	⬇️

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[sebastianbergmann]

Not sure that support for old versions of PHP and PHPUnit needs to be kept. What's keeping us from bumping the major version and removing support for PHP 5.3-5.6 and PHPUnit < 6.

[SpacePossum]

More than happy to bump to PHP7 and PHPUnit 6 :)
Therefor I propose to wrap up this one on the current 1.x, giving the current users stuck on PHP > 7 the optimization and bug fixes on the current code.

Than I work on adding the line numbers; I've played around with it and one way or the other it would require a BC break. Maybe it would be nice to (after this PR) add the line numbers and bump to PHP7 & PHPUnit 6 and by that start working on a 2.x release of the diff. WDYT?

[sebastianbergmann]

Lets get this merged first and then we'll see. Let me know when this is ready to be merged.

[SpacePossum]

I think this one is ready :)