Speedup psalm test with only diff

[menno-ll]

In the composer test script, testing with psalm takes a long time. We can make it faster for example if we only check changed files. maybe we should consider that. See command options with /vendor/bin/psalm --help or https://github.com/vimeo/psalm/blob/master/docs/running_psalm/command_line_usage.md

Command outputs:

➜  Clarkson-Core git:(feature/psalm) vendor/bin/psalm --threads=8 --diff --diff-methods --show-info=false
Scanning files...
Analyzing files...

------------------------------
No errors found!
------------------------------

Checks took **7.10** seconds and used 313.776MB of memory
No files analyzed
➜  Clarkson-Core git:(feature/psalm) vendor/bin/psalm --show-info=false                                  
Scanning files...
Analyzing files...

------------------------------
No errors found!
------------------------------

Checks took **13.76** seconds and used 361.078MB of memory
Psalm was able to infer types for 85.2657% of the codebase
➜  Clarkson-Core git:(feature/psalm) 

Originally posted by @menno-ll in https://github.com/level-level/Clarkson-Core/pull/151#issuecomment-500860612