rust-filepatterns: unescape comment character property
There were multiple issues in the original implementation:
a. the local variable "line" dropped soon after replace_slice() applied
b. replace_slice() was noop since br"\#".len() != b"#"
This patch uses bytes::Regex::replace_all() since it seems the simplest way
to replace bytes of arbitrary length, and I don't think we have to avoid
using Regexp here.
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