Tips

$ git clone https://path/repo/git
Initialized empty Git repository in /path/to/location

(gnome-ssh-askpass:4449): Gtk-WARNING **: cannot open display:

$ unset SSH_ASKPASS
$ git clone https://path/repo/git

Initialized empty Git repository in /path/to/location
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W: GPG error: https://repo.com xenial Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY KEY_ID#
W: The repository 'https://repo.com/ubuntu/testing xenial Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: There is no public key available for the following key IDs:
KEY_ID#

=> sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys KEY_ID#