Fix bug in format-diff.sh (#6772)

Summary:
Nasty bug in which more/different changes would be applied than
those shown to user
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6772

Test Plan: manual

Reviewed By: siying

Differential Revision: D21304604

Pulled By: pdillinger

fbshipit-source-id: 7e20740e513c9c300d1522511290a025b35abedc

build_tools/format-diff.sh

@@ -121,7 +121,7 @@ if [ -z "$uncommitted_code" ]
 then
   git diff -U0 "$FORMAT_UPSTREAM_MERGE_BASE" | $CLANG_FORMAT_DIFF -i -p 1
 else
-  git diff -U0 HEAD^ | $CLANG_FORMAT_DIFF -i -p 1
+  git diff -U0 HEAD | $CLANG_FORMAT_DIFF -i -p 1
 fi
 echo "Files reformatted!"