Ignore .shelf directory
Ignore .shelf directory

This directory is used by Intellij for shelved
files.

Also simplified the .gitignore file by removing
duplicate entries.

Closes #2205 from HappyRay/ignore-shelf-dir

shirshanka/gobblin,ibuenros/gobblin,jack-moseley/gobblin,abti/gobblin,ibuenros/gobblin,ibuenros/gobblin,abti/gobblin,jinhyukchang/gobblin,abti/gobblin,jack-moseley/gobblin,linkedin/gobblin,shirshanka/gobblin,jinhyukchang/gobblin,linkedin/gobblin,jinhyukchang/gobblin,jack-moseley/gobblin,jinhyukchang/gobblin,ibuenros/gobblin,arjun4084346/gobblin,linkedin/gobblin,ibuenros/gobblin,jinhyukchang/gobblin,aditya1105/gobblin,linkedin/gobblin,jack-moseley/gobblin,aditya1105/gobblin,arjun4084346/gobblin,shirshanka/gobblin,aditya1105/gobblin,jack-moseley/gobblin,abti/gobblin,abti/gobblin,arjun4084346/gobblin,linkedin/gobblin,abti/gobblin,aditya1105/gobblin,shirshanka/gobblin,aditya1105/gobblin,linkedin/gobblin,aditya1105/gobblin,arjun4084346/gobblin,jinhyukchang/gobblin,ibuenros/gobblin,shirshanka/gobblin,arjun4084346/gobblin