Fix `git svn` `rebase` & `dcommit` if top-level HEAD directory exist

When a file (or a directory) called HEAD exists in the working tree,
internal calls git svn makes trigger "did you mean a revision or a
path?" ambiguity check.

    $ git svn rebase
    fatal: ambiguous argument 'HEAD': both revision and filename
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'
    rev-list --first-parent --pretty=medium HEAD: command returned error: 128

Explicitly disambiguate by adding "--" after the revision.
Signed-off-by: NSlava Kardakov <ojab@ojab.ru>
Reviewed-by: NJeff King <peff@peff.net>
Acked-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -831,7 +831,7 @@ sub dcommit_rebase {
 sub cmd_dcommit {
 	my $head = shift;
 	command_noisy(qw/update-index --refresh/);
-	git_cmd_try { command_oneline(qw/diff-index --quiet HEAD/) }
+	git_cmd_try { command_oneline(qw/diff-index --quiet HEAD --/) }
 		'Cannot dcommit with a dirty index.  Commit your changes first, '
 		. "or stash them with `git stash'.\n";
 	$head ||= 'HEAD';
@@ -1932,7 +1932,7 @@ sub cmt_sha2rev_batch {
 sub working_head_info {
 	my ($head, $refs) = @_;
 	my @args = qw/rev-list --first-parent --pretty=medium/;
-	my ($fh, $ctx) = command_output_pipe(@args, $head);
+	my ($fh, $ctx) = command_output_pipe(@args, $head, "--");
 	my $hash;
 	my %max;
 	while (<$fh>) {