util.system: avoid buffering of subprocess output when it is piped
util.system() copies subprocess' output through pipe if output file is not
stdout. Because a file iterator has internal buffering, output won't be
flushed until enough data is available. Therefore, it could easily miss
important messages such as "waiting for lock".
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1