patman: Use the full commit hash for 'git checkout'

Even with the initial 8 characeters of the hash we will sometimes get a
collision. Use the full hash.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index 6a4de36..fb79a1e 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -70,7 +70,7 @@
     if commits:
         for upto in range(0, len(series.commits), options.step):
             commit = series.commits[upto]
-            print '   ', col.Color(col.YELLOW, commit.hash, bright=False),
+            print '   ', col.Color(col.YELLOW, commit.hash[:8], bright=False),
             print commit.subject
     print
     for arg in why_selected: