Revive Repository#notifyIndexChanged()

Revive Repository#notifyIndexChanged()

e9e150fdd24d (Store in IndexChangedEvent if it was caused by JGit
itself, 2018-05-13) modified Repository#notifyIndexChanged to take a
boolean argument to indicate whether the index change happened under
the current process's control or externally, for use by EGit.  In
other words, the function signature changed from

	public abstract void notifyIndexChanged();

to

	public abstract void notifyIndexChanged(boolean internal);

Callers outside JGit itself notifying a Repository about index changes
are expected to be rare, so this is not very disruptive to them.  In
most cases they would be notifying about changes that they made
themselves, so treating their notifyIndexChanged() calls as
notifyIndexChanged(true) should be relatively safe.

Implementors have the opposite problem: adding the new "abstract void
notifyIndexChanged(boolean)" method means they are obligated to
override it.  Add a default implementation that calls their existing
override of notifyIndexChanged() to make their migration easier.

The main downside is that authors of new Repository subclasses that
do not realize they need to override notifyIndexChanged would end up
with a default implementation which calls notifyIndexChanged(true),
in turn calling notifyIndexChanged() again and so on, resulting in
StackOverflowException.  Add an implementors' note to the class
Javadoc to avoid this issue.  A followup commit will force
implementors to adapt to the new API by changing the methods to

	@Deprecated
	public final void notifyIndexChanged() {
		notifyIndexChanged(true);
	}

	public abstract void notifyIndexChanged(boolean internal);

Change-Id: I7d014890ee19abf283ea824d9baa9044bfdde130
Signed-off-by: Jonathan Nieder <jrn@google.com>