It can be useful to produce patches from older commits, for example to share
commits from the keyring branch between the main Guix repository and
guix-maintenance. The previous 'true' value prevented that with a fatal error
message.
* etc/git/gitconfig [format] <useAutoBase>: Set to whenAble.
Change-Id: Ib32407dd3695dcadcb16855b1b87147fabc2aca7