This change adds documentation and demos missing for the flag
--inject_current used in git new-branch.

Bug: 1167943
Change-Id: Id6305b876ade2c47eb7435a5edd055ae4fe2f323
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2648615Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>