error on gzipped GFAs

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• vg autoindex --gfa will error if the filename seemed gzipped

Description

Saw #4856 and, didja know, there's a utility function for that :)

This doesn't actually detect the gzip-ness from the file content, just from filenames. It's a band-aid. Likely effective though.