shell=True is too dangerous to allow; there are unfiltered user inputs

There are all sorts of unfiltered user inputs like tag and branch names in
source repos.  If those names are fed into popen calls that use shell=True,
that opens up a wide range of exploits.  All core operations should never
use shell=True.

hooks/pre-commit

@@ -129,4 +129,8 @@ for f in $RB_FILES; do
 	fi
 done
 
+if grep --line-number 'shell=True' fdroidserver/[a-ce-z]*.py; then
+    err "shell=True is too dangerous, there are unfiltered user inputs!"
+fi
+
 exit 0