Script for running uncrustify for modified files.

Comments in the script explain how to use it manually or automatically.
The script uses the git filter attribute to indentify which files to run
uncrustify for.  This makes it possible to configure a git filter for
running uncrustify instead of using the script.  This is also described
in the script comments.

The script is easily extensible to do other types of automatic
reformatting, e.g., to check/add copyright headers.

Also contains a pre-commit hook for running uncrustify.  Currently only
checks the formatting; does not yet automatically reformat anything, as
it is a bit unclear what is the best way to do that.

Related to #845.

Change-Id: I389f2ebf43182d4827d1ec10cb6c1332bd4bf8de