git: don't redundantly pass "--" to git_sha1

git_sha1 already puts "--" before its arguments, so

    git_sha1_file($dir, 'doc/index.mdwn')

would have incorrectly invoked

    git rev-list --max-count=1 HEAD -- -- doc/index.mdwn

If there is no file in the wiki named "--", that's harmless, because
it merely names the latest revision in which either "--" or
"doc/index.mdwn" changed. However, it could return incorrect results
if there is somehow a file named "--".

IkiWiki/Plugin/git.pm

@@ -578,7 +578,7 @@ sub rcs_find_changes ($) {
 sub git_sha1_file ($$) {
 	my $dir=shift;
 	my $file=shift;
-	return git_sha1($dir, "--", $file);
+	return git_sha1($dir, $file);
 }
 
 sub git_sha1 ($@) {