Support setting `gitlab_shell_git_timeout` via Helm Chart
<!-- NOTICE: This Issue tracker is for the GitLab Helm chart, not the GitLab Rails application. Support: Please do not raise support issues for GitLab.com on this tracker. See https://about.gitlab.com/support/ --> ## Summary The `gitlab_shell_git_timeout` setting controls repository clone/fetch timeouts and is especially relevant for Geo deployments with limited bandwidth. [We have documentation for how to change this setting in an Omnibus deployment](https://archives.docs.gitlab.com/18.7/administration/geo/replication/troubleshooting/synchronization_verification/#error-fetch-remote-signal-terminated-context-deadline-exceeded-at-exactly-3-hours), but there isn't currently a way to configure the timeout for the Helm chart. This work item is to request this value be added to the Helm chart to allow configuration by users that need it for their Geo deployments. ## Configuration used (Please provide a _sanitized_ version of the configuration used wrapped in a code block (```yaml)) ```yaml appConfig: gitlab_shell: git_timeout: 28800 ``` ## Current behavior Configuring `git_timeout` in the yaml has no effect on the instance. ## Expected behavior Configuring `git_timeout` should apply the updated timeout setting. ## Versions - Chart: Any =< 19.10.2
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