buildman: Ignore conflicting tags

Tags like Series-version are normally expected to appear once, and with a
unique value. But buildman doesn't actually look at these tags. So ignore
conflicts.

This allows bulidman to build a branch containing multiple patman series.

Reported-by: Steve Rae <srae@broadcom.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/func_test.py

@@ -79,6 +79,7 @@ Date:   Thu Aug 14 16:48:25 2014 -0600
     Series-changes: 7
     - Add new patch to fix the 'reverse' bug
 
+    Series-version: 8
 
     Change-Id: I79078f792e8b390b8a1272a8023537821d45feda
     Reported-by: York Sun <yorksun@freescale.com>
@@ -156,6 +157,8 @@ Date:   Fri Aug 22 15:57:39 2014 -0600
     Series-changes: 9
     - Add new patch to avoid changing the order of tags
 
+    Series-version: 9
+
     Suggested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
     Change-Id: Ib1518588c1a189ad5c3198aae76f8654aed8d0db
 """]