Fix spurious compile error

Fix spurious compile error

From time to time, I would get this error:

[...]
sed: -e expression #8, char 41: Unterminated `s' command
make: *** [git-add--interactive] Error 1

Turns out that the function WriteMakefile() called in Makefile.PL
outputs the message "Writing perl.mak for Git" to stdout! Thus,
the output of "make -C perl -s --no-print-directory instlibdir"
would be prefixed by that message whenever Makefile.PL was newer
than perl.mak.

This is fixed by redirecting stdout to stderr in Makefile.PL.

Signed-off-by: Johannes E. Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>