repotest: refactor shellcheck slightly

Refactor the shellcheck implementation so that:
   1. Files are checked by extension if their `file` type doesn't
      match a known list.

   Some shell scripts do not use the proper shebang and end up not being
   detected by `file`; attempt to revert to extension in that case.

   2. Facilitate easier addition of multiple linting tools for other
      file types.

   A follow up commit will add 'json' linting with only a few changes.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5ce60432c4bd123d9d9c14466505c2d4b2477959

0 files changed, 0 insertions, 0 deletions