RunCmd() fails when invoked program writes a large amount to stdout (ex. greater than 770K)

[singweeMsftGitHub]

This issue was discovered as part of a hang observed while running a Project Mu LicenseCheck.

The LicenseCheck plug-in calls RunCmd as follows:

return_buffer = StringIO()
params = "diff --unified=0 origin/master HEAD"
RunCmd("git", params, outstream=return_buffer)

RunCmd was found to hang if the diff generates a large amount of text (in my case, greater than 770K).

The following article suggests why this might be the case:
https://thraxil.org/users/anders/posts/2008/03/13/Subprocess-Hanging-PIPE-is-your-enemy/