transport-helper: read helper response with strbuf_getline()

transport-helper: read helper response with strbuf_getline()

Our implementation of helpers never use CRLF line endings, and they
do not depend on the ability to place a CR as payload at the end of
the line, so this is essentially a no-op for in-tree users.  However,
this allows third-party implementation of helpers to give us their
line with CRLF line ending (they cannot expect us to feed CRLF to
them, though).

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