.gitconfig: update config (url.pushInsteadOf, remove deprecated declarations)

Copy parts of https://github.com/arthurdarcet/dotfiles/blob/master/.gitconfig

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Add optional .shell/local file to execute host-specific instructions

For example such file may export proxy environment, define local aliases, etc.

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
.profile: remove buggy zsh code which is never automatically executed

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Replace .xsession with XDG autostart files

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
.tmux.conf: disable mouse, keeping options commented in config file

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>