split the section "Stage and Unstage" into two

Split the section "Stage and Unstage" (a subsection of "Acting") into
two subsections "Stage" and "Unstage".  I admit this is completely
unnecessary but I have to satisfy my ocd.   :-)
Merge branch 'ocd'

Conflicts:
	magit.el
move magit-grep to section "Miscellaneous Commands"
Merge branch 'grep'
magit-read-file-from-rev: fallback to HEAD if REVISION is nil

Because some of its callers fail to provide a non-nil value for the
mandatory REVISION argument when HEAD is detached, explicitly fallback
to HEAD in `magit-read-file-from-rev' for now.  That isn't optimal but
it works well enough as a temporary kludge.