contrib/subtree: fix spelling of accidentally

Noticed with Lucas De Marchi's codespell tool.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/t/t7900-subtree.sh

@@ -419,7 +419,7 @@ test_expect_success 'add main-sub5' '
 test_expect_success 'split for main-sub5 without --onto' '
         # also test that we still can split out an entirely new subtree
         # if the parent of the first commit in the tree is not empty,
-        # then the new subtree has accidently been attached to something
+	# then the new subtree has accidentally been attached to something
         git subtree split --prefix subdir2 --branch mainsub5 &&
         check_equal ''"$(git log --pretty=format:%P -1 mainsub5)"'' ""
 '