status: improve parallel execution stability

The status command runs a bunch of jobs in parallel, and each one
is responsible for writing to stdout directly.  When running many
noisy jobs in parallel, output can get intermingled.  Pass down a
StringIO buffer for writing to so we can return the entire output
as a string so the main job can handle displaying it.  This fixes
interleaved output as well as making the output stable: we always
display results in the same project order now.  By switching from
map to imap, this ends up not really adding any overhead.

Bug: https://crbug.com/gerrit/12231
Change-Id: Ic18b07c8074c046ff36e306eb8d392fb34fb6eca
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297242


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>