Additionally, defined the NoteMap.getNote method which returns a Note
instance.  These changes were necessary to enable implementation of
the NoteMerger interface (the merge method needs to instantiate a
Note) and to enable direct use of NoteMerger which expects instances
of Note class as its paramters.  Implementing creation of code review
summary notes in Gerrit [1] will make use of both of these features.

[1] https://review.source.android.com/#change,20045

Change-Id: I627aefcedcd3434deecd63fa1d3e90e303b385ac
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

We will need to iterate over all notes of a NoteMap, at least this will be
needed for testing purposes. This change also implied making the Note class
public.

Change-Id: I9b0639f9843f457ee9de43504b2499a673cd0e77
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>

The NoteMap makes it easy to read a small notes tree as created by
the `git notes` command in C Git.  To make the initial implementation
simple a notes tree is read recursively into a map in memory.
This is reasonable if the application will need to access all notes,
or if there are less than 256 notes in the tree, but doesn't behave
well when the number of notes exceeds 256 and the application
doesn't need to access all of them.

We can later add support for lazily loading different subpaths,
thus fixing the large note tree problem described above.

Currently the implementation only supports reading.  Writing notes
is more complex because trees need to be expanded or collapsed at
the exact 256 entry cut-off in order to retain the same tree SHA-1
that C Git would use for the same content.  It also needs to retain
non-note tree entries such as ".gitignore" or ".gitattribute" files
that might randomly appear within a notes tree.  We can also add
writing support later.

Change-Id: I93704bd84ebf650d51de34da3f1577ef0f7a9144
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

Replace the old crude event listener system with a much more generic
implementation, patterned after the event dispatch techniques used
in Google Web Toolkit 1.5 and later.

Each event delivers to an interface that defines a single method,
and the event itself is what performs the delivery in a type-safe
way through its own dispatch method.

Listeners are registered in a generic listener list, indexed by
the interface they implement and wish to receive an event for.
Delivery of events is performed by looping through all listeners
implementing the event's corresponding listener interface, and using
the event's own dispatch method to deliver the event.  This is the
classical "double dispatch" pattern for event delivery.

Listeners can be unregistered by invoking remove() on their
registration handle.  This change therefore requires application
code to track the handle if it wishes to remove the listener at a
later point in time.

Event delivery is now exposed as a generic public method on the
Repository class, making it easier for any type of message to
be sent out to any type of listener that has registered, without
needing to pre-arrange for type-safe fireFoo() methods.

New event types can be added in the future simply by defining a
new RepositoryEvent subclass and a corresponding RepositoryListener
interface that it dispatches to.  By always adding new events through
a new interface, we never need to worry about defining an Adapter
to provide default no-op implementations of new event methods.

Change-Id: I651417b3098b9afc93d91085e9f0b2265df8fc81
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Per CQ 3448 this is the initial contribution of the JGit project
to eclipse.org.  It is derived from the historical JGit repository
at commit 3a2dd9921c.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>