git-p4: add read_pipe_text() internal function

git-p4: add read_pipe_text() internal function

The existing read_pipe() function returns an empty string on
error, but also returns an empty string if the command returns
an empty string.

This leads to ugly constructions trying to detect error cases.

Add read_pipe_text() which just returns None on error.

Signed-off-by: Luke Diamand <redacted>
Signed-off-by: Junio C Hamano <redacted>