Ignore backup files

[Dreamsorcerer]

From #448 (comment)

I get an error when it fails to git remove some hidden backup files on my computer.

Personally, I'd be fine if it simply skipped any files which are not committed, but I see there are planned changes to go in the other direction, in which case simply ignoring common backup files would be fine too.

The backup files in my case all end with ~. This is a common pattern on Linux systems and are treated as hidden files by a lot of software (similar to starting with .).

[adiroiban]

If someone will send a good PR to ignore any files that are not in git, we can merge it :)

To me it makes sense to ignore the files that are not in git.

towncrier is all about gathering the files from multiple branches.

[Dreamsorcerer]

This would contradict the changes in #357, so if we're saying that we definitely won't handle untracked files in future, then that should be closed.

If we do go this way, then a quick search suggests a simple solution could be to run git ls-tree --full-tree -r --name-only HEAD [path-to-change-fragments], which will give us a list of tracked files. Then we could simply use that instead of browsing the directory, I suppose.

[Dreamsorcerer]

Which would also solve #448 easily.

[Dreamsorcerer]

So, I reckon just replacing

towncrier/src/towncrier/_builder.py

Line 104 in 0b023fa

files = os.listdir(section_dir)

With a git call to git ls-tree --full-tree --name-only HEAD {section_dir} may be enough, though I've not tried anything yet.

[adiroiban]

Sound very good. Thanks.

Anythig that has good automated tests and doesn't break the current test suite should be good enogh to be merged.

We are already calling git in various places, so calling it one more time snould not be a problem.

Also, as we are going to call git rm on the files, I think that it makes sense to also obtain the list of files using git.