Introduce shallow gitignore parsing for better performance

[marionebl]

• See Improve performance #50
• Too slow on large projects xojs/xo#234
• Slow when linting a single file xojs/xo#212

This changes the .gitignore handling to only scan for ignore files at $CWD/.gitignore, which results in better performance of globby runs.

Previously globby scanned for **/.gitignore, which increased execution time in relation to the number of files in `cwd.

This does not change the ignore filter to be applied before glob tasks are created.

Considerations

• Should nested .gitignore files be supported?
• Probably should move benchmarking for .gitignore handling here
• Should ignore files be searched from ${gitRoot}/ instead? (How to handle non-git dirs then?)

Details

Tested via https://github.com/pixelass/ava-xo-test:

git clone https://github.com/pixelass/ava-xo-test.git
git clone https://github.com/sindresorhus/globby.git

cd globby 
git checkout perf-shallow-gitignore
npm install

cd ../ava-xo-test
npm install
# Prevent xo from finding the local (old) copy
mv node_modules/xo/cli.js node_modules/xo/_cli.js

# Local
# node --inspect --inspect-brk ./node_modules/xo/_cli.js 

# Development
# node --inspect --inspect-brk ../xo/cli.js 

Before

~/Documents/oss/ava-xo-test master*
λ time ./node_modules/.bin/xo

  4 errors
       58.69 real        52.60 user         7.43 sys

The area with red borders signifies time spent in getGitIgnoreFilter

CPU profile before

After

~/Documents/oss/ava-xo-test master*
λ time node ../xo/cli.js

  5 errors
       42.90 real        39.16 user         5.81 sys

CPU profile after

[marionebl]

Obsolete as per v8