Add start_sha to UserCommitFilesRequestHeader

gitlab-ce#60609
4 jobs for webide-commit-use-correct-parent in 1 minute and 53 seconds
Status Job ID Name Coverage
  Test
passed #236042712
build

00:01:44

passed #236042713
check-grpc-proto-clients

00:01:40

passed #236042714
danger-review

00:01:01

passed #236042711
test

00:01:53