Merge branch 'mn/sideband-no-ansi' into maint

Tools that read diagnostic output in our standard error stream do
not want to see terminal control sequence (e.g. erase-to-eol).
Detect them by checking if the standard error stream is connected
to a tty.

* mn/sideband-no-ansi:
  sideband.c: do not use ANSI control sequence on non-terminal

sideband.c

@@ -30,7 +30,7 @@ int recv_sideband(const char *me, int in_stream, int out)
 
 	memcpy(buf, PREFIX, pf);
 	term = getenv("TERM");
-	if (term && strcmp(term, "dumb"))
+	if (isatty(2) && term && strcmp(term, "dumb"))
 		suffix = ANSI_SUFFIX;
 	else
 		suffix = DUMB_SUFFIX;