If `repo start foo` fails due to uncommitted and unmergeable changes
in a single project, we have switched half of the projects over to
the new target branches, but didn't on the one that failed to move.
This change improves the situation by doing three things differently:
- We keep going when we encounter an error, so other projects
that can successfully switch still switch.
- We ignore projects whose current branch is already on the
requested name; they are logically already setup.
- We checkout the branch if it already exists, rather than
trying to recreate the branch.
Bug: REPO-22
Signed-off-by: Shawn O. Pearce <sop@google.com>
