Merge 'pk/rebase-in-c-6-final'

This final patch flips the switch and makes the builtin rebase the
default. The old, Unix shell scripted version can still be called via

	git -c rebase.useBuiltin=false rebase [...]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/rebase.c

@@ -54,7 +54,7 @@ static int use_builtin_rebase(void)
 	cp.git_cmd = 1;
 	if (capture_command(&cp, &out, 6)) {
 		strbuf_release(&out);
-		return 0;
+		return 1;
 	}
 
 	strbuf_trim(&out);