avoid collision with master branch in check format

Summary:
The new local branch specified with -b cannot be called master. Use tmp prefix to avoid name collision.
Closes https://github.com/facebook/rocksdb/pull/2600

Differential Revision: D5442944

Pulled By: maysamyabandeh

fbshipit-source-id: 4a623d9b21d6cc01bee812b2799790315bdf5f6e

tools/check_format_compatible.sh

@@ -85,7 +85,7 @@ done
 checkout_flag=${1:-"master"}
 
 echo == Building $checkout_flag debug
-https_proxy="fwdproxy:8080" git checkout github_origin/$checkout_flag -b $checkout_flag
+https_proxy="fwdproxy:8080" git checkout github_origin/$checkout_flag -b tmp-$checkout_flag
 make clean
 make ldb -j32
 compare_base_db_dir=$test_dir"/base_db_dir"