roll_dep.py: Support upstream branch being origin/main.
roll_dep.py: Support upstream branch being origin/main.

Bug: None
Change-Id: I93df760f8d5431e55ef337c05634e7f26ad69fd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2512921
Auto-Submit: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Reviewed-by: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools