os-release: ensure 'dirty' really means dirty.

When using --git-dir, git no longer automatically determines the
working tree and thus always assumes the tree is dirty unless the
--work-tree option is also passed.

Fixes openbmc/openbmc#820.

Change-Id: I3af6941e0562bf665290e57c2b7a80459385fefa
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/common/recipes-core/os-release/os-release.bbappend b/common/recipes-core/os-release/os-release.bbappend
index d91823d..1ac195f 100644
--- a/common/recipes-core/os-release/os-release.bbappend
+++ b/common/recipes-core/os-release/os-release.bbappend
@@ -1,7 +1,8 @@
 def run_git(d, cmd):
         try:
                 oeroot = d.getVar('COREBASE', True)
-                return bb.process.run("git --git-dir %s/.git %s" %(oeroot, cmd))[0].strip('\n')
+                return bb.process.run("git --work-tree %s --git-dir %s/.git %s"
+                                      % (oeroot, oeroot, cmd))[0].strip('\n')
         except:
                 pass