[PATCH] JavaHL: Support commit callback across method invocations

In case of delta editor, commit callback is provided at the time of the editor
creation and used during the close_edit() call. For that there is a need to
keep a global reference to the underlying java object so that it does not get
GCed meanwhile.

Attached please find the patch that adds such capabilities.

Thank you,

Vladimir

[[[
JavaHL: Support keeping global reference to the callback java object for cases
when callback is being used across method calls

[ in subversion/bindings/javahl/native ]

* CommitCallback.cpp,
  CommitCallback.h
  (CommitCallback, ~CommitCallback): Add handling of additional parameter and
    state when requesting a global reference to be kept
]]]