Fix the stdin diff examples of the documentation

[lyrixx]

Q	A
Branch	v2.x
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets

81a6f9a replaced git diff with git diff --raw in the stdin examples, as gitlib needs the raw
information to parse a diff. But --raw alone prints the raw lines only, without the patch: the
input has no diff --git, so StdInFiles reads it as a list of files, each raw line being a path:

$ git diff --staged --raw | php vendor/bin/grumphp run
- :100644 100644 7898192 9ad2ebb M	a.txt exceeded the maximum size of 1.

With --patch too, like the hooks already do (diff --raw -r -p ...), GrumPHP gets the files:

$ git diff --staged --raw --patch | php vendor/bin/grumphp run
- a.txt exceeded the maximum size of 1.

[veewee]

Thanks!