feat: Support filenames containing # in ignore and remove files

[twpayne]

Fixes #4206.

[halostatue]

I was just going to say that this is a good fix, but if you added a second check for ^\\# and stripped the leading \\, we could support files that start with # by doing \#foo. This isn't necessary for any other use of # because of the "beginning or whitespace".

[twpayne]

Ah, but then we'd need to start worrying about files whose names begin with a backslash.

[halostatue]

Ah, but then we'd need to start worrying about files whose names begin with a backslash.

Only if they start with \#, which would be even more rare.

[twpayne]

There remains the issue of handling arbitrary filenames, e.g. "this # is a \ filename" is a valid filename. Rather than try to handle all possible cases, I think the current solution is sufficient for now.