Various fixes to sign-my-commit:

- more consistent use of quotes
 - add additional feedback when signature is already detected
 - add --force option
 - only select latest signature if multiple signature headers found

Fix squash-my-commit when signature not requested

squash-my-commits

@@ -30,13 +30,14 @@ usage()
 
 do_push=0
 verify_only=0
+do_sign=0
 
 for arg
 do
     case "$arg" in
 
         -S)
-            do_sign='-S'
+            do_sign=1
             ;;
         --push)
             do_push=1
@@ -156,7 +157,12 @@ $(git log --oneline HEAD ^dn42registry/master)"
 
 # and finally squash
 git reset --soft dn42registry/master
-git commit "$do_sign" -m "$comment"
+if [ "$do_sign" -eq 1 ]
+then
+    git commit -S -m "$comment"
+else
+    git commit -m "$comment"
+fi
 
 # show what happened
 echo '---'