Update 'git push' command in GettingStarted guide

'git push' command, without any other arguments, can do different
things depending on the local configuration of Git. This patch
updates the 'git push' command with extra arguments to be more
resilient to any local configuration.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D79964

Author: dcaballe

llvm/docs/Phabricator.rst

@@ -212,10 +212,10 @@ the following:
 
 ::
 
-  git pull --rebase origin master
+  git pull --rebase https://github.com/llvm/llvm-project.git master
   git show # Ensure the patch looks correct.
   ninja check-$whatever # Rerun the appropriate tests if needed.
-  git push
+  git push https://github.com/llvm/llvm-project.git HEAD:master
 
 Or