update: add stricter checking when updating repo index using rsync
update: add stricter checking when updating repo index using rsync

rsync uses the modification time and size of the file when deciding whether
to update a file.  These are relatively easy to control in malicious code,
so instead make rsync use a full MD5 checksum when decided whether the
index needs to be updated.  I suppose we could add an option to use
checksum checking on all files, but since the signed repo already provides
a checksum check, it seems not worth the added load on the process.

Also, renamed 'index' to 'indexxml' to make it clear what is the XML and
what is the JAR.

f-droid/fdroid-server,matlink/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,matlink/fdroidserver,f-droid/fdroidserver,matlink/fdroidserver,fdroidtravis/fdroidserver,OneEducation/AppUniverse_Server,matlink/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,matlink/fdroidserver,f-droid/fdroid-server,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroidserver,OneEducation/AppUniverse_Server,f-droid/fdroid-server,f-droid/fdroidserver,OneEducation/AppUniverse_Server