sparse-checkout: optionally turn off cone mode

While it _is_ true that cone mode is the default nowadays (mainly for
performance reasons: code mode is much faster than non-cone mode), there
_are_ legitimate use cases where non-cone mode is really useful.

Let's add a flag to optionally disable cone mode.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

action.yml

@@ -58,6 +58,10 @@ inputs:
       Do a sparse checkout on given patterns.
       Each pattern should be separated with new lines
     default: null
+  sparse-checkout-cone-mode:
+    description: >
+      Specifies whether to use cone-mode when doing a sparse checkout.
+    default: true
   fetch-depth:
     description: 'Number of commits to fetch. 0 indicates all history for all branches and tags.'
     default: 1