`options.cwd` should use the canonical pathname

[ehmicky]

TextExclude.shouldInstrument(filename) returns false when options.relativePath is true and filename is not inside options.cwd.

However:

• when options.cwd is a symlink and is passed with its symlink unresolved
• but filename resolves the symlink (through fs.realpath() for example)
• then shouldInstrument() will return false even though it should return true.

This is likely to happen since:

• the filename passed by nyc always has symlinks resolved because that's how module._compile() works
• however nyc user might pass a options.cwd that has an unresolved symlink.

Real-life example: running the following on Travis on Mac OS:

const os = require('os');
const execa = require('execa');

const cwd = os.tmpdir();
await execa.shell(`nyc --cwd ${cwd}`)

Travis Mac OS environment:

• returns /var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T with os.tmpdir()
• but this directory is a symlink for /private/var/folders/nz/vv4_9tw56nv9k3tkvyszvwg80000gn/T
• as a consequence all filename passed to testExclude.shouldInstrument() will start with /private/var/..., while options.cwd() will start with /var/..., even though it's the same folder.

One possible solution would be to run fs.realpath() on both filename and options.cwd() before comparing them.