pager: preserve Hg's exit code (and fix Windows support) (issue3225) This changes how the pager extension invokes the pager. Prior to this change, the extension would fork Hg and exec the pager in the parent process. This loses Hg exit code, and it doesn't work on Windows. Now the pager is invoked using the subprocess library, and an atexit handler is registered that makes Hg wait for the pager to exit before it exits itself. Note that if you exit the pager before Hg is done running, you'll get an exit code of 255, which is caused by Python blowing up due to a broken pipe. If you set pager.quiet=True, you'll get the OS-level return code of 141.