vimrc: limit git commit messages to 76 characters.

When doing git log -p, the text is indented by four characters, making
80-character lines unpleasant when reviewing code.  Using 76 characters
means this text will fit appropriately in this space.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Merge branch 'master' into work