add magit-add-change-log-entry{ replacing -no-option}

Add command `magit-add-change-log-entry' replacing the less flexible
`magit-add-change-log-entry-no-option'.  This is done by duplicating
the (trivial) prefix argument handling from `add-log-change-log-entry'.
magit-add-change-log-entry-other-window use magit...entry

Define `magit-add-change-log-entry-other-window' using
`magit-add-change-log-entry' instead of interactively calling
`add-change-log-entry-other-window'.

This makes the use of the macro `magit-visiting-file-item' unnecessary.
We will take advantage of that in the next commit.
remove macro magit-visiting-file-item

Remove the macro `magit-visiting-file-item'.  Since the last commit only
one use of the macro remains.  By doing the same thing *without* the use
of a macro we remove a blackbox.
magit-add-change-log-entry: don't use a marker

In `magit-add-change-log-entry' stop using a marker when remembering the
buffer and position is enough.  The use of a marker would have been okay
if it had been disposed of after use, but that wasn't the case.
Merge branch 'pr/change-log'

Conflicts:
	magit.el