[rewrite] Do not leave .zs-old files around

[probonopd]

When the file was already 100% downloaded and up to date and the user then tries to update it, a .zs-old file is left behind:

me@host:~$ ls
appimaged-x86_64.AppImage	  Desktop    Downloads	Pictures
appimaged-x86_64.AppImage.zs-old  Documents  Music	Videos
me@host:~$ rm appimaged-x86_64.AppImage.zs-old 
me@host:~$ ls
appimaged-x86_64.AppImage  Desktop  Documents  Downloads  Music  Pictures  Videos
me@host:~$ Downloads/appimageupdate-x86_64.AppImage appimaged-x86_64.AppImage
Starting update...
Updating from generic server via ZSync
Update URL: https://github.com/AppImage/AppImageKit/releases/download/continuous/appimaged-x86_64.AppImage.zsync
zsync2: appimaged-x86_64.AppImage found, using as seed file
zsync2: Reading seed file: appimaged-x86_64.AppImage
zsync2: Usable data from seed files: 100.000000%
zsync2: Renaming temp file
zsync2: Fetching remaining blocks
zsync2: Verifying downloaded file
zsync2: checksum matches OK
zsync2: used 245760 local, fetched 0
100% done...
Update successful!
me@host:~$ ls
appimaged-x86_64.AppImage	  Desktop    Downloads	Pictures
appimaged-x86_64.AppImage.zs-old  Documents  Music	Videos

[TheAssassin]

Rewrite does cleanup its backup files as of 4b1e3a3.

[probonopd]

Still an issue if you already have the new version downloaded. #36

[TheAssassin]

That issue originates from your idea to keep the old files and create a new one with the new filename. When there's a file called like the new filename, it will be backed up to avoid deleting it.

A "fix" for this would probably be to make the update check look whether there's such a new file, and whether it is equal to the server file. This avoids unnecessary updates.

[probonopd]

Just having two identical filenames does not mean that the content is necessarily the same. If your proposal takes this into account, then I am all for it.

[TheAssassin]

The idea is to apply the selected update check to this file, too, yes.

[TheAssassin]

This has probably been resolved since AppImageUpdate checks for updates before starting an actual update now. Please confirm.

[probonopd]

Did not encounter it recently, hence closing. Should reopen if the problem appears again. Thanks.

[probonopd]

AppImageUpdate-382-b61bee3-x86_64.AppImage still leaves around .zs-old files.

Let's do some check before exiting to clean up any remaining .zs-old files.