run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
adding a
? a
adding a
A a
A a
? b
A a
A b
% should fail
b already tracked!
A a
A b
% should fail
a already tracked!
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
created new head
merging a
warning: conflicts during merge.
merging a failed!
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg up --clean' to abandon
M a
? a.orig
% should fail
a already tracked!
M a
? a.orig
% issue683
R a
? a.orig
M a
? a.orig
c does not exist!
d does not exist!
M a
A c
? a.orig