Fix quiet mode in git-clang-format

Quiet mode is very useful for scripting, when only the diff format output is
required, or no output if not formatting is needed.
In case of no modified files, git-clang-format will output to screen even though
the quiet mode enabled.

This patch changes this behavior, so if quiet flag passes in - no output will be
available, even if no modified files exists.

Differential Revision: https://reviews.llvm.org/D85485