gitconfig: include a platform specific file

Plaform specific options are included through
.gitconfig_platform_specific which is a symlink to the the appropiate
file .gitconfig_osx or .gitconfig_linux.

The symlink is created by the create_links*.sh scripts
vimrc: Do not remember position for commit messages
Add git-overwritting command

It shows who are the original authors of the lines that you just edited.
Useful to find who should review your changes.