sideband.c: Get rid of ANSI sequence for non-terminal shell

Some git tools such as GitExtensions for Windows use environment variable TERM=msys which causes the weird ANSI sequence shown for the messages returned from server-side hooks

See https://github.com/gitextensions/gitextensions/issues/1313 for more details

NOTE: I considered to cover the case that a pager has already been started. But decided that is probably not worth worrying about here, though, as we shouldn't be using a pager for commands that do network communications (and if we do, omitting the magic line-clearing signal is probably a sane thing to do).