Add support for partial checkout filters (#1396)

* added filter option & tests

* added build file

* fix test oversight

* added exit 1

* updated docs to specify override

* undo unneeded readme change

* set to undefined rather than empty string

* run git config in correct di

---------

Co-authored-by: Cory Miller <13227161+cory-miller@users.noreply.github.com>

README.md

@@ -75,8 +75,12 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
     # Default: true
     clean: ''
 
+    # Partially clone against a given filter. Overrides sparse-checkout if set.
+    # Default: null
+    filter: ''
+
     # Do a sparse checkout on given patterns. Each pattern should be separated with
-    # new lines
+    # new lines.
     # Default: null
     sparse-checkout: ''