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closes #13

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