pr-booster - careful with large git invocations

[Julius Davies [bit-booster.com]]

The rebase/squash/amend/cherry-pick/revert buttons run "git for-each-ref" and "git log" calls under the hood to pre-fill some of the UI dialogs (e.g., target branches for cherry-pick and commit message for squash).

In pathological cases these commands could consume a lot of memory (temporarily). Protections should be put in place to guard against this.