Exclude external subdirectory from clang-format pre-commit hook

Author: robertodr

.githooks/pre-commit-clang-format

@@ -72,7 +72,8 @@ patch=`mktemp -p /tmp ${prefix}.XXXXXX.patch`
 $DELETE_OLD_PATCHES && rm -f /tmp/$prefix*.patch
 
 # create one patch containing all changes to the files
-git diff-index --cached --diff-filter=ACMR --name-only $against -- | while read file;
+# exclude files under external
+git diff-index --cached --diff-filter=ACMR --name-only $against -- . ':!external' | while read file;
 do
     # ignore file if we do check for file extensions and the file
     # does not match any of the extensions specified in $FILE_EXTS