Gogs: Overwriting critical files results in a denial of service

Gogs is an open source self-hosted Git service. Prior to 0.14.0, as an authorized user, an intruder can dictate the value which is passed to the git diff command which, together with bypassing the filtering of the passed value, allows the user to bypass the target directory and write the result of the comparison to any arbitrary path. This vulnerability is fixed in 0.14.0.