pager: do not dup2 stderr if it is already redirected

An earlier commit 61b80509 (sending errors to stdout under $PAGER,
2008-02-16) avoided losing the error messages that are sent to the
standard error when $PAGER is in effect by dup2'ing fd 2 to the pager.
his way, showing a tag object that points to a bad object:

    $ git show tag-foo

would give the error message to the pager.  However, it was not quite
right if the user did:

    $ git show 2>error.log tag-foo

i.e. use the pager but store the errors in a separate file.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

pager.c

@@ -102,7 +102,8 @@ void setup_pager(void)
 
 	/* original process continues, but writes to the pipe */
 	dup2(pager_process.in, 1);
-	dup2(pager_process.in, 2);
+	if (isatty(2))
+		dup2(pager_process.in, 2);
 	close(pager_process.in);
 
 	/* this makes sure that the parent terminates after the pager */