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.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda