Matching for excluded_patterns in .index.yml

[andyw8]

As mentioned in #1827

Let's say we have a file abc.rb that we want to exclude from indexing.

If we try exclude it in index.yml with:

excluded_patterns:
  - "abc.rb"

or

excluded_patterns:
  - "/abc.rb"

then it will still be indexed. Instead we need to write:

excluded_patterns:
  - "**/abc.rb"

which is not so obvious, and is different from other exclusion mechanism such as .gitignore.