coccinelle: convert hashcpy() with null_sha1 to hashclr()

hashcpy with null_sha1 as the source is equivalent to hashclr.  In
addition to being simpler, using hashclr may give the compiler a chance
to optimize better.  Convert instances of hashcpy with the source
argument of null_sha1 to hashclr.

This transformation was implemented using the following semantic patch:

@@
expression E1;
@@
-hashcpy(E1, null_sha1);
+hashclr(E1);

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-submodule-config.c

@@ -49,7 +49,7 @@ int main(int argc, char **argv)
 		path_or_name = arg[1];
 
 		if (commit[0] == '\0')
-			hashcpy(commit_sha1, null_sha1);
+			hashclr(commit_sha1);
 		else if (get_sha1(commit, commit_sha1) < 0)
 			die_usage(argc, argv, "Commit not found.");