Fix ignoring input files for symlink reasons (#4222)

This relates to #4015, #4161 and the behaviour of os.getcwd()

Black is a big user of pathlib and as such loves doing `.resolve()`,
since for a long time it was the only good way of getting an absolute
path in pathlib. However, this has two problems:

The first minor problem is performance, e.g. in #3751 I (safely) got rid
of a bunch of `.resolve()` which made Black 40% faster on cached runs.

The second more important problem is that always resolving symlinks
results in unintuitive exclusion behaviour. For instance, a gitignored
symlink should never alter formatting of your actual code. This kind of
thing was reported by users a few times.

In #3846, I improved the exclusion rule logic for symlinks in
`gen_python_files` and everything was good.

But `gen_python_files` isn't enough, there's also `get_sources`, which
handles user specified paths directly (instead of files Black
discovers). So in #4015, I made a very similar change to #3846 for
`get_sources`, and this is where some problems began.

The core issue was the line:
```
root_relative_path = path.absolute().relative_to(root).as_posix()
```
The first issue is that despite root being computed from user inputs, we
call `.resolve()` while computing it (likely unecessarily). Which means
that `path` may not actually be relative to `root`. So I started off
this PR trying to fix that, when I ran into the second issue. Which is
that `os.getcwd()` (as called by `os.path.abspath` or `Path.absolute` or
`Path.cwd`) also often resolves symlinks!
```
>>> import os
>>> os.environ.get("PWD")
'/Users/shantanu/dev/black/symlink/bug'
>>> os.getcwd()
'/Users/shantanu/dev/black/actual/bug'
```
This also meant that the breakage often would not show up when input
relative paths.

This doesn't affect `gen_python_files` / #3846 because things are always
absolute and known to be relative to `root`.

Anyway, it looks like #4161 fixed the crash by just swallowing the error
and ignoring the file. Instead, we should just try to compute the actual
relative path. I think this PR should be quite safe, but we could also
consider reverting some of the previous changes; the associated issues
weren't too popular.

At the same time, I think there's still behaviour that can be improved
and I kind of want to make larger changes, but maybe I'll save that for
if we do something like #3952

Hopefully fixes #4205, fixes #4209, actual fix for #4077