Optimize ls-remote with --heads

Only looking at heads branches so optimize the query
by passing --heads option

Change-Id: I97a870b8e2964b90b77bdce333db1aea4115e5c2
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 0f2040e..998e58c 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -65,12 +65,12 @@
 
   local tip
   # Need to continue if branch not found, hence || true at end
-  tip=$(git ls-remote "https://github.com/openbmc/${package}" |
+  tip=$(git ls-remote --heads "https://github.com/openbmc/${package}" |
         grep "refs/heads/$BRANCH" | awk '{ print $1 }' || true)
 
   # If specific branch is not found then try master
   if [ ! -n "$tip" ]; then
-    tip=$(git ls-remote "https://github.com/openbmc/${package}" |
+    tip=$(git ls-remote --heads "https://github.com/openbmc/${package}" |
          grep "refs/heads/master" | awk '{ print $1 }')
   fi