contrib/git-svn: allow rebuild to work on non-linear remote heads

Because committing back to an SVN repository from different
machines can result in different lineages, two different
repositories running git-svn can result in different commit
SHA1s (but of the same tree).  Sometimes trees that are tracked
independently are merged together (usually via children),
resulting in non-unique git-svn-id: lines in rev-list.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

contrib/git-svn/git-svn.perl

@@ -850,11 +850,23 @@ sub assert_revision_unknown {
 	}
 }
 
+sub trees_eq {
+	my ($x, $y) = @_;
+	my @x = safe_qx('git-cat-file','commit',$x);
+	my @y = safe_qx('git-cat-file','commit',$y);
+	if (($y[0] ne $x[0]) || $x[0] !~ /^tree $sha1\n$/
+				|| $y[0] !~ /^tree $sha1\n$/) {
+		print STDERR "Trees not equal: $y[0] != $x[0]\n";
+		return 0
+	}
+	return 1;
+}
+
 sub assert_revision_eq_or_unknown {
 	my ($revno, $commit) = @_;
 	if (-f "$REV_DIR/$revno") {
 		my $current = file_to_s("$REV_DIR/$revno");
-		if ($commit ne $current) {
+		if (($commit ne $current) && !trees_eq($commit, $current)) {
 			croak "$REV_DIR/$revno already exists!\n",
 				"current: $current\nexpected: $commit\n";
 		}