clean P2P protocol shutdown on EOF

Avoids "git-annex-shell: <stdin>: hGetChar: end of file"
being displayed by the test suite, due to the way it
runs git-annex-shell without using ssh.

git-annex-shell over ssh was not affected because git-annex hangs up the
ssh connection and so never sees the error message that git-annnex-shell
probably did emit.

This commit was sponsored by Ryan Newton on Patreon.

Utility/SimpleProtocol.hs

@@ -113,6 +113,8 @@ dupIoHandles = do
  - This implementation is not super efficient, but as long as the Handle
  - supports buffering, it avoids reading a character at a time at the
  - syscall level.
+ -
+ - Throws isEOFError when no more input is available.
  -}
 getProtocolLine :: Handle -> IO (Maybe String)
 getProtocolLine h = go (32768 :: Int) []