Improve slightly the pre-commit git hook usage of the auto-format helper (#117326)

The default mode does not provide a way to see the actual failure of the
formatters without modifying the code. Instead offer the user the option
to rerun with a `FORMAT_HOOK_VERBOSE=1` environment variable to print
the actual formatting diff.

Author: joker-eph

llvm/utils/git/code-format-helper.py

@@ -318,7 +318,7 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
 def hook_main():
     # fill out args
     args = FormatArgs()
-    args.verbose = False
+    args.verbose = os.getenv("FORMAT_HOOK_VERBOSE", False)
 
     # find the changed files
     cmd = ["git", "diff", "--cached", "--name-only", "--diff-filter=d"]
@@ -338,6 +338,9 @@ def hook_main():
             print(f"Couldn't find {fmt.name}, can't check " + fmt.friendly_name.lower())
 
     if len(failed_fmts) > 0:
+        print(
+            "Pre-commit format hook failed, rerun with FORMAT_HOOK_VERBOSE=1 environment for verbose output"
+        )
         sys.exit(1)
 
     sys.exit(0)