[Feature] Change the base gerrit_git_dir to the appropriate commit as indicated by a patch

[WLIU0628]

Sometimes, patches sent in the mailing list have different base commits than the master branch of Linux. As such, the master branch should be checked out to whatever's specified by the patch.

[dlatypov]

Picking a short small commit as an example: https://lore.kernel.org/linux-kselftest/20201102232304.2735187-1-dlatypov@google.com/

Close to the end of the message, there's a string like

base-commit: 495023e4e49e4b7dee35928800bf0317276576c1
-- 

This only shows up for the first message.

This is documented here: https://git-scm.com/docs/git-format-patch#_base_tree_information
There's also options fo "prerequisite-patch-id", which we might want to support at some point as well.