_merge_process: don't try to create path up to offset prefix

_merge_process: don't try to create path up to offset prefix

A merge conflict went unnoticed, which introduced a regression where
portage would try to create all dirs from root up to the file to merge.
In Prefix, users often don't have permission to write there, and it
isn't necessary.  The search hence can just start from the offset
prefix, since that space is controlled by Portage and should be
writable.