sparse-checkout: reject arguments in cone-mode that look like patterns

In sparse-checkout add/set under cone mode, the arguments passed are
supposed to be directories rather than gitignore-style patterns.
However, given the amount of effort spent in the manual discussing
patterns, it is easy for users to assume they need to pass patterns such
as
   /foo/*
or
   !/bar/*/
or perhaps they really do ignore the directory rule and specify a
random gitignore-style pattern like
   *.c

To help catch such mistakes, throw an error if any of the positional
arguments:
  * starts with any of '/!'
  * contains any of '*?[]'

Inform users they can pass --skip-checks if they have a directory that
really does have such special characters in its name.  (We exclude '\'
because of sparse-checkout's special handling of backslashes; see
the MINGW test in t1091.46.)

Reviewed-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1091-sparse-checkout-builtin.sh

@@ -673,7 +673,7 @@ test_expect_success BSLASHPSPEC 'pattern-checks: escaped characters' '
 	git -C escaped reset --hard $COMMIT &&
 	check_files escaped "a deep folder1 folder2 zbad\\dir zdoes*exist" zglob[!a]? &&
 	git -C escaped sparse-checkout init --cone &&
-	git -C escaped sparse-checkout set zbad\\dir/bogus "zdoes*not*exist" "zdoes*exist" "zglob[!a]?" &&
+	git -C escaped sparse-checkout set --skip-checks zbad\\dir/bogus "zdoes*not*exist" "zdoes*exist" "zglob[!a]?" &&
 	cat >expect <<-\EOF &&
 	/*
 	!/*/