To do so, I slightly changed the behavior of unannex. Now in fast mode, it
only makes a hard link when the annexed file's link count is 1. This avoids
unannexing 2 files with the same content in fast mode from hard linking
them together. (One will end up hard linked to the annex, which the docs
warn about.)
With that change, uninit can simply always run unannex in fast mode. Since
.git/annex/objects is being blown away anyway, there's no worry in this
case about a hard link pointing into it causing an annexed object to be
modified.
