git-svn: require working HTTPS for all Subversion URLs

Subversion does not verify each commit as strongly as git does, so HTTPS is
really important.  Also, there is the possibility of injecting code into
`fdroid checkupdate` calls if plain HTTP is used.

fdroidserver/common.py

@@ -1004,6 +1004,9 @@ class vcs_gitsvn(vcs):
             else:
                 remote = self.remote
 
+            if not remote.startswith('https://'):
+                raise VCSException(_('HTTPS must be used with Subversion URLs!'))
+
             gitsvn_args.extend(['--', remote, self.local])
             p = self.git(gitsvn_args)
             if p.returncode != 0: