Git config allows value setting from stdin

When setting values in the git config, the value is part of the arguments for execution. This potentially leaks the value through logging, or other programs like ps.

Prior to this change, there was no option to do this. This change adds the --stdin to be combined with --add. When passed, the value cannot be passed and is read through stdin.

Signed-off-by: Zeger-Jan van de Weg git@zjvandeweg.nl