add fake argument to MigrationManager

[dantownsend]

Related to #515

[codecov-commenter]

Codecov Report

Merging #516 (f178c36) into master (a89c2fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #516   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files         104      104           
  Lines        7010     7011    +1     
=======================================
+ Hits         6362     6363    +1     
  Misses        648      648           

Impacted Files	Coverage Δ
piccolo/apps/migrations/auto/migration_manager.py	94.58% <100.00%> (+0.01%)	⬆️
piccolo/apps/migrations/commands/forwards.py	98.59% <100.00%> (ø)

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