BUG: Not shrinking commands themselves

[dubzzz]

Before this PR, fc.commands was only able to shrink the last command.

In the attached commit I updated the test Should shrink up to the shortest failing commands list so that inc and dec can increase more than ones per command execution. With the current implementation it resulted in a poorly shrunk counterexample:

Should shrink up to the shortest failing commands list

    Expected value to equal:
      "inc[1],check[1]"
    Received:
      "inc[1350995096],check[1]"

[coveralls]

Coverage increased (+0.4%) to 97.417% when pulling 33dc9ee on bug/cmds-partial-shrink into 3c1592f on master.