Don't try to determine interactivity if ui() called with interactive=False.
WSGI applications are not supposed to refer to sys.stdin. In ed6df6b1c29a,
hgweb and hgwebdir were fixed to pass interactive=False to their ui()'s, but
sys.stdin.isatty() was still called by the ui objects. This change makes sure
only the ui.fixconfig() method will call ui.isatty() (by making the
ui._readline() method, which is currently only called from ui.prompt(),
private). ui.fixconfig() is changed to let config files override the initial
interactivity setting, but not check isatty() if interactive=False was
specified in the creation of the ui.
#!/bin/sh
# Test for changeset ba7c74081861
# (update dirstate correctly for non-branchmerge updates)
hg init a
cd a
echo a > a
hg add a
hg commit -m a
cd ..
hg clone a b
cd a
hg mv a b
hg commit -m move
echo b >> b
hg commit -m b
cd ../b
hg pull ../a
hg update