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Experimental support for 'global_variables' Processor attrs

- can optionally define 'global_variables' similar to input/output vars
- when these are present, AutoPackager.verify() will ensure any
required variables are present and also search these keys in the
com.github.autopkg prefs domain
- removing hardcoded 'MUNKI_REPO' supported pref
Cleanup some unused variables.
Allow short GitHub-ish repo names to be given instead of full URLs for ‘repo-‘ commands

- non-GitHub URLs pass through unmodified
- ‘user/reciperepo’ expands to ‘https://github.com/user/reciperepo'
- ‘reciperepo’ expands to 'https://github.com/autopkg/reciperepo'
Remove explicit ‘global_variables’ and instead load _all_ pref keys in com.github.autopkg into the env.

- get_all_prefs() now loads all prefs that exist in both ~/L/P and /L/P, and initializes AutoPackager’s
  env with these as before
- no more ‘SUPPORTED_PREFS’
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repo- commands: fix full URLs having "https://github.com/autopkg/" prepended
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Release version 0.2.5.
Bumping to v0.2.6 for development.
GitHub releases API no longer in preview
Substitute call() with check_call()
build release out of new Git clone rather than current working tree to avoid conflicts
Fix for FoundationPlist on 10.6.x
Release version 0.2.6.
Bumping to v0.2.7 for development.
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Fix refactoring bug when readPlist encounters an error
Fixing long string wrapping in several processor's descriptions and input/output variable descriptions.
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