merge toplevel sections "Acting (1)" and "Acting (2)"

Until recently the toplevel section "Log Edit Mode" was between these
two sections, but that was moved to a separate file.

To merge these two sections another toplevel that was between the two
had to be made a subsection.  In the process that section "ChangeLog"
was renamed to "Commit".  In the future it will also contain a command
`magit-commit', which will be a wrapper around `magit-log-edit' or the
respective function from `git-commit-mode'.
allow using magit-add-log during rebase

Like `magit-log-edit' this function detected an progress rebase and
offered to continue that instead of committing.  When the user reply
was "no" this function acted as if that means "no I want to do
anything at all".  `magit-log-edit' already understands it correctly
"no thanks, I really do want to commit".

Change this function to behave the same way.  Because this function
used `magit-log-edit' internally the test for an in-progress rebase
can be removed here.  There is some code before the call to
`magit-log-edit' but that only collects some information, has no side
effects, and is inexpensive.