do not redownload unchanged InRelease files

do not redownload unchanged InRelease files

Before we download the 'new' InRelease file the old file will be moved
out of the way with the name 'foobar_InRelease.reverify', so if no
partial file for the 'new' file exists take the modification time from
this reverify file, so that if we get an IMS hit for the InRelease file
we can move back the reverify file as new file rather than downloading
the 'new' file even though we already have it.

We do the same for Release files and this happened to work until the
reverify renaming was corrected for InRelease files.