Fix non-interactive usage

Because:

* Currently, the hook will always prompt for user input if warnings are
  detected.
* This prevents usage in non-interactive contexts such as some Git GUI
  clients and CI.

Notes:

* I don't love the test specific `FAKE_TTY` featuring in the hook
  itself, but I can't think of a way around this right now and I think
  the non-interactive support is worth it.

Credit @ssbarnea with the solution, ref:
#18

Author: tommarshall

hook.sh

@@ -284,6 +284,11 @@ while true; do
 
   display_warnings
 
+  # if non-interactive don't prompt and exit with an error
+  if [ ! -t 1 ] && [ -z ${FAKE_TTY+x} ]; then
+    exit 1
+  fi
+
   # Ask the question (not using "read -p" as it uses stderr not stdout)
   echo -en "${BLUE}Proceed with commit? [e/y/n/?] ${NC}"