Speed up 'repo start' by removing some forks

Its quite common for most projects to be matching the current
manifest revision, as most developers only modify one or two projects
at any one time.  We can speed up `repo start foo` (that impacts
the entire client) by performing most of the branch creation and
switch operations in pure Python, and thus avoid 4 forks per project.

Signed-off-by: Shawn O. Pearce <sop@google.com>
2 files changed