$ git pull upstream master
remote: Counting objects: 36, done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 19 (delta 6), reused 18 (delta 5)
Unpacking objects: 100% (19/19), done.
From https://github.com/openmrs/openmrs-core
 * branch            master     -> FETCH_HEAD
fatal: Unable to create 'D:/Projects/openMRS/openmrs-core/.git/index.lock': File
 exists.

If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.