Fix a couple of problems in changelog generating functions

Fix a couple of problems in changelog generating functions

* lisp/vc/diff-mode.el (diff-add-log-current-defuns): If there is a
scan-error when calling end-of-defun, go to end of hunk.  This can
easily happen since we are calling end-of-defun on a partial code
fragment from a diff.
* lisp/vc/log-edit.el (log-edit-generate-changelog-from-diff): Bind
display-buffer-overriding-action around the log-edit-show-diff call
only.  Otherwise, it can affect, for example, debugger windows
triggered by the diff-add-log-current-defuns call.