Backport #12102
this was thought to be due to timeouts, however on closer look this
appears to be due to the Close() function of the BlameReader hanging
with a blocked stdout pipe.
This PR fixes this Close function to:
* Cancel the context of the cmd
* Close the StdoutReader - ensuring that the output pipe is closed
Further it makes the context of the `git blame` command a child of the
request context - ensuring that even if Close() is not called, on
cancellation of the Request the blame is command will also be cancelled.
Fixes#11716Closes#11727
Signed-off-by: Andrew Thornton <art27@cantab.net>
20c2bdf86b