Function getChangedPackagesBetweenRefs

Finds all packages that had been changed between two refs in the repo under cwd

executes a git diff $fromRef...$toRef to get changes given a merge-base

further explanation with the three dots:

git diff [--options] <commit>...<commit> [--] [<path>...]

   This form is to view the changes on the branch containing and up to
   the second <commit>, starting at a common ancestor of both
   <commit>. "git diff A...B" is equivalent to "git diff
   $(git-merge-base A B) B". You can omit any one of <commit>, which
   has the same effect as using HEAD instead.
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