builtin-bundle.c - use stream buffered input for rev-list

git-bundle create on cygwin was nearly unusable due to 1 character
at a time (unbuffered) reading from an exec'ed process. Fix by using
fdopen to get a buffered stream.

Results for "time git bundle create test.bdl v1.0.3..v1.5.2" are:

before this patch:
cygwin         linux
real    1m38.828s      0m3.578s
user    0m12.122s      0m2.896s
sys     1m28.215s      0m0.692s

after this patch:
real    0m3.688s       0m2.835s
user    0m3.075s       0m2.731s
sys     0m1.075s       0m0.149s

Signed-off-by: Mark Levedahl <mdl123@verizon.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>