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Before this change apkbuild-lint finds an extra error in the test data:

    <file>:3: prefix custom variable with _: foo=
    <file>:3: variable set to empty string: foo=

After this change only the desired error is found:

    <file>:3: prefix custom variable with _: foo=example

closes #11

The amount of spaces before the line number can vary between different
cat implementations. Account for that in the sed pattern that parses the
output.

Test 'empty global variable can be removed' is currently failing.

pkgname=

Works with ash btw

closes #7

closes #1

- Don't match strings that don't start at the beginning of a line, it
  has a legitimate use-case of strings used in subpackages that need to
  be empty
- But match empty strings that declare themselves empty by having "" as
  their value, this was previously ignored because it was how
  subpackages would declare themselves but it would miss lots of
  subpackages