audit: only check previous formula version.

For calculating the stable/devel versions this should be sufficient as
it's looking at `origin/master` so for a e.g. unmerged pull request this
will stop complaining about mistakes outside the pull request itself.

This will silence all warnings for historic version mistakes (i.e.
before these audit checks were all enabled) which is normally a bad
thing but as this case would rely on modifying history to complete is a
good one.

Library/Homebrew/dev-cmd/audit.rb

@@ -661,7 +661,7 @@ class FormulaAuditor
     return unless formula.tap.git? # git log is required
     return if @new_formula
 
-    fv = FormulaVersions.new(formula, max_depth: 10)
+    fv = FormulaVersions.new(formula, max_depth: 1)
     attributes = [:revision, :version_scheme]
 
     attributes_map = fv.version_attributes_map(attributes, "origin/master")