This fixes intermingling of parallel jobs and simplifies the code
by switching to subprocess.run. This also provides stable output
in the order of projects by returning the output as a string that
the main loop outputs.
This drops support for interactive commands, but it's unclear if
anyone was relying on that, and the default behavior (-j2) made
that unreliable. If it turns out someone still wants this, we can
look at readding it.
Change-Id: I7555b4e7a15aad336667292614f730fb7a90bd26
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297482
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
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