$ git pull upstream master
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.