Allow users to filter subdirs of their current dir.

This also greatly simplifies the backtrace formatter;
the system_exclusion_patterns approach made it more
complicated, as did always having an inclusion_pattern.

We only need the current directory as an inclusion_pattern
when it matches one of the built-in exclusion patterns.

Note that this was the approach that we originally had in #843;
it got lost in the refactoring in #1062 (af0b271c92ad2a3336eb70be7bf2c0de4325a568).
While it seemed like a simpler approach to always put the current dir
in the inclusion_patterns, in practice, this caused a sequence
of whack-a-mole regressions:

- #1328 (fixed by #1329)
- #1604 (fixed by #1616)
- #1705 (fixed by this commit)

I'm hopeful that returning to only including the current dir
in `inclusion_patterns` if it matches a built-in exclusion pattern
will solve these issues once and for all.
Merge pull request #1708 from rspec/fix-1705

Allow users to filter subdirs of their current dir.