Symbols for index-modifying cards, remove index/mv level for now

Plus better descriptions for pull/push/fetch.

levels/index/checkout

@@ -1,5 +1,5 @@
 title = Checking out files from the index
-cards = add reset checkout-file commit
+cards = add reset-file checkout-file commit
 
 [description]
 

levels/index/mv

@@ -1,26 +0,0 @@
-title = Rename a file in the next commit
-cards = add reset checkout-file mv commit
-
-[description]
-
-Other times, you might want to rename a file in the next commit. Use
-
-    git mv [file] [new name]
-
-for that. The effect is very similar as if you had created a copy with a new name, and removed the old version.
-
-[setup]
-
-echo a > a
-echo SPECIAL > b
-echo x > c
-git add .
-git commit -m "Initial commit"
-echo x > a
-echo b >> b
-git add b
-
-[win]
-
-# Make a commit where you rename the file b to "x".
-test "$(git ls-tree --name-only main)" = "$(echo -e "a\nc\nx")"

levels/index/reset

@@ -1,5 +1,5 @@
 title = Resetting files in the index
-cards = add reset commit
+cards = add reset-file commit
 
 [description]
 

levels/index/rm

@@ -1,5 +1,5 @@
 title = Delete a file in the next commit
-cards = add reset checkout-file rm file-delete commit
+cards = add reset-file checkout-file rm file-delete commit
 
 [description]
 

levels/index/sequence

@@ -4,4 +4,3 @@ add
 reset
 checkout
 rm
-mv