refile: Never change the current folder

Refile is different to all other tools, as its +folder arguments
are destination folders, not source folders (it even supports
multiple of them). A source folder can be specified with -src.
Refile used to change the current folder to the source folder.
It does not do so anymore.

Most often the current folder is used as the source folder; this is
the default. Hence changing the current folder is a no-op. But if
one refiles from some other source folder, changing to that folder
is seldom useful. Consider the scenario where one wants to collect
the last messages of a set of folders in some other folder. The
old behavior required to know where you are to collect the messages,
because after the first refile you'd be somewhere else:

	for i in one two three ; do
		refile -src +$i l +lastmsgs
	done
	# now we are in +three, hence go back where we were
	folder +lastmsgs

The new behavior allows to move the messages just here (@), no
matter where that is:

	for i in one two three ; do
		refile -src +one l @
	done
	# now we still are where we were

Refiling actions do no more affect the current folder, hence one
may securely refile with different -src +folder many times in one
row, without having to care for changing locations.