[WIP][HOTFIX][SPARK-4123]: Fix bug in PR dependency (all deps. removed issue)

We're seeing a bug sporadically in the new PR dependency comparison test whereby it notes that *all* dependencies are removed. This happens when the current PR is built, but the final, sorted, dependency file is left blank. I believe this is an error either in the way the `git checkout` calls have been or an error within the `mvn` build for that PR (again, likely related to the `git checkout`). As such I've set the checkouts to now force (with `-f` flag) which is more in line with what Jenkins currently does on the initial checkout.

Setting this as a WIP for now to trigger the build process myriad times to see if the issue still arises.

Author: Brennon York <brennon.york@capitalone.com>

Closes #5443 from brennonyork/HOTFIX2-SPARK-4123 and squashes the following commits:

f2186be [Brennon York] added output for the various git commit refs
3f073d6 [Brennon York] removed the git checkouts piping to dev null
07765a6 [Brennon York] updated the diff logic to reference the filenames rather than hardlink
e3f63c7 [Brennon York] added '-f' to the checkout flags for git
710c8d1 [Brennon York] added 30 minutes to the test benchmark

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