tools: add `make format-cpp` to run clang-format on C++ diffs

This patch adds a `make format-cpp` shortcut to the Makefile
that runs clang-format on the C++ diffs, and a
`make format-cpp-build` to install clang-format from
npm.

To format staged changes:

```
$ make format-cpp
```

To format HEAD~1...HEAD (latest commit):

```
$ CLANG_FORMAT_START=`git rev-parse HEAD~1` make format-cpp
```

To format diff between master and current branch head (master...HEAD):

```
$ CLANG_FORMAT_START=master make format-cpp
```

Most of the .clang-format file comes from running

```
$ clang-format --dump-config --style=Google
```

with clang-format built with llvm/trunk 328768 (npm version 1.2.3)

The clang-format version is fixed because different version of
clang-format may format the files differently.

PR-URL: https://github.com/nodejs/node/pull/21997
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index 1d901b3a69..255a4c5a4b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 !test/fixtures/**/.*
 !tools/node_modules/**/.*
 !tools/doc/node_modules/**/.*
+!.clang-format
 !.editorconfig
 !.eslintignore
 !.eslintrc.js