format_code.py changed to print the diff when not formatting files inline

[dconeybe]

This PR modifies format_code.py to print a unified diff of incorrectly-formatted files when both -noformat_file and -verbose are specified. This can make it easier to correct formatting problems based on the output from GitHub Actions without having to run clang-format yourself. This is especially useful if the version of clang-format on the GitHub Actions runners differs from that on your local machine, as they may produce different results.

Here is an example output of format_code.py:

$ python3 scripts/format_code.py -noformat_file -verbose -f example.cc                  
Found 1 file(s). Checking their format.
  - Requires reformatting: "example.cc"
------ BEGIN FORMATTING DIFF OF example.cc
---
+++
@@ -1,9 +1,9 @@
+#include <string>
 #include <vector>
-#include <string>

 std::vector<std::string> GetStrings() {
-    std::vector<std::string> v;
+  std::vector<std::string> v;
   v.emplace_back("string1");
-  v.emplace_back( "string2" );
+  v.emplace_back("string2");
   return v;
 }
------ END FORMATTING DIFF OF example.cc