subrepo: make stdin for svn a pipe for non-interactive use (issue2759)
This certainly can't hurt, so go ahead and do it, potentially along
with --non-interactive if that flag is safe for the given subcommand.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a