Allows using HEAD in performanceTest.wls and fixes it on CI

[maxitg]

Changes

• Closes #573.
• Allows using HEAD as an argument to ./performanceTest.wls.
• Fixes CI to use HEAD instead of (an incorrect) @HEAD.

Comments

• ./performanceTest.wls was broken on CI because it fails with HEAD and silently fails with @HEAD by running the tests on the same branch again (so the performance report is always at zero).

Examples

• Test HEAD against master:

$ ./performanceTest.wls master HEAD 2
Testing master

Testing 286a74794bb15a89addc2c697a7d66124b10de0e

Single-input rule                       -5.5 ± 0.6 %
Medium rule                             -1.2 ± 2.7 %
Sequential rule                         -0.2 ± 0.6 %
Large rule                              -1.9 ± 1.2 %
Exponential-match-count rule            -1. ± 4. %
CA emulator                             0. ± 10. %

• Test a previous commit vs. its predecessor:

$ ./performanceTest.wls @~2 @^ 2
Testing 4122bf6ede4d58207923360009e096eb04239409

Testing 3e71dee78255a937f3bb67b5dc25322d03cfb763

Single-input rule                       -43.2 ± 2.0 %
Medium rule                             -32.0 ± 1.5 %
Sequential rule                         -25.3 ± 1.0 %
Large rule                              -21.5 ± 3.0 %
Exponential-match-count rule            -17.4 ± 1.8 %
CA emulator                             -2. ± 9. %

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