`gts check/fix` uses a list of files created by tslint about gts

[ofrobots]

I'm unable to reproduce this on your plugin-types branch. You will have to do some initial investigation yourself and report back.

[kjin]

The issue stems from me including generated git-ignored source files, which are being checked. I didn't realize that I had already fixed them locally, and CI kept failing because it was regenerating these source files.

This makes me think of two improvements we can make to avoid this problem (despite the questionable usage of generated source files in the linked PR, in general it's a use case we still want to support):

• gts check and gts fix should show a list of files that clang-format takes issue with, if not the fixes that should be made/have been made themselves. I already talked about this in #60 as being a low-priority improvement but I now think this would be extremely useful.
• We should not use the list of files generated by tslint for formatting (this is the current behavior). It seems like Linter.getFileList(Linter.createProgram('...')) gets a list of files in a source dependency graph, making its return value a longer list than the list of files matching the glob specified in tsconfig.json. This is useful for linting, but undesirable for formatting.