run clean branch baseline only once

bin/c

 #!/bin/sh
 
 set -e
 
 BRANCH=$(git branch | grep \* | cut -d" "  -f 2)
 CLEAN=${CLEAN-clean}
 SAMPLES=100
 
 if [ $BRANCH == $CLEAN ]; then
     echo "Comparing $BRANCH to itself."
     exit 1
 fi
 
-git checkout $CLEAN
-./gyp_skia >/dev/null
-ninja -C out/Release nanobench
-out/Release/nanobench $@ --samples $SAMPLES -v 2> $CLEAN.log
+if [ ! -f $CLEAN.log ]; then
+    git checkout $CLEAN
+    ./gyp_skia >/dev/null
+    ninja -C out/Release nanobench
+    out/Release/nanobench $@ --samples $SAMPLES -v 2> $CLEAN.log
+fi
 
 git checkout $BRANCH
 ./gyp_skia >/dev/null
 ninja -C out/Release nanobench
 out/Release/nanobench $@ --samples $SAMPLES -v 2> $BRANCH.log
 
 compare $CLEAN.log $BRANCH.log