Handle branch names containing dependency group

[CloudNStoyan]

Fixes #489

When extracting the directory from the branch name of a Dependabot PR that uses groups, the branch name will only include the group name instead of dependency names as described below:

This issue is caused by https://github.com/dependabot/fetch-metadata/blob/main/src/dependabot/update_metadata.ts#L52
That code is expecting the branch name to contain the dependency name when for grouped updates it will only contain the group name, and because all the dependency names contain multiple /s it is failing to get the directory name.
This is similar to #493 and #494, but it is not addressed by my pr #495.
Originally posted by @dreid in #489 (comment)

I have fixed that and I have added 3 tests to test the new logic

I also had to edit this test because it depended on the broken group logic

fetch-metadata/src/main.test.ts

Line 212 in 06ea45a

test('it supports returning information about grouped updates', async () => {

Here the test checks if the directory is set to / while the PR is a group one and the branch name is this: dependabot/docker/gh-base-image/docker-1234566789 which is incorrect, here the directory should be gh-base-image