Fix and test handling of backslashes in hooks (#3909)

* Update hooks to use `read -r` and `printf`

This portably avoids any interpretation of backslash escapes by the
shell.

* Support running make_tc_task elsewhere

* Add a test for backslashes in task descriptions

scripts/hooks/on-add

@@ -10,7 +10,7 @@ read -r new_task
 # Output:
 # - JSON, modified or unmodified.
 # - Optional feedback/error.
-echo "$new_task"
+printf "%s\n" "$new_task"
 echo 'on-add'
 
 # Status:

scripts/hooks/on-add.the

@@ -2,11 +2,11 @@
 
 read -r new_task
 
-if (echo "$new_task" | grep -qE '[tT]eh');
+if (printf "%s" "$new_task" | grep -qE '[tT]eh');
 then
-  new_task=$(echo "$new_task" | sed -r 's/([tT])eh/\1he/g')
+  new_task=$(printf "%s" "$new_task" | sed -r 's/([tT])eh/\1he/g')
   echo "Auto-corrected 'teh' --> 'the'"
 fi
 
-echo "$new_task"
+printf "%s" "$new_task"
 exit 0

scripts/hooks/on-modify

@@ -12,8 +12,8 @@ read -r modified_task
 # Output:
 # - JSON, modified or unmodified.
 # - Optional feedback/error.
-echo "$modified_task"
-echo 'on-modify'
+printf "%s" "$modified_task"
+printf "%s" 'on-modify'
 
 # Status:
 # - 0:     JSON accepted, non-JSON is feedback.