[tools] `rev_sdk_deps.dart` output commit message
[tools] `rev_sdk_deps.dart` output commit message

This CL changes the output of the tool to have the desired commit
message, including title and reference to the script.

Also scan repos in parallel, reducing execution time from 19 to 4
seconds on my machine.

Change-Id: Ia5fe75c21dd3b217038337e31161d2b88d22026c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265282
Reviewed-by: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Auto-Submit: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk