Update atomic constant

The atomic feature is now cooking in -next in git-core. Very rarely
features are ejected from the the next branch in git-core, so I consider
it reasonable to come up with this patch now to make the 2 implementations
interoperable.

Change-Id: I806a8ae3c045ca5936f69cb903baf9b99ee39181
Signed-off-by: Stefan Beller <sbeller@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java

 	 *
 	 * @since 3.6
 	 */
-	public static final String CAPABILITY_ATOMIC = "atomic-push"; //$NON-NLS-1$
+	public static final String CAPABILITY_ATOMIC = "atomic"; //$NON-NLS-1$
 
 	/**
 	 * The client expects a status report after the server processes the pack.