mergestate: raise exception if otherctx is accessed but _other isn't set
We don't want to inadvertently return the workingctx (self._repo[None]).
#require no-icasefs
test file addition with colliding case
$ hg init repo1
$ cd repo1
$ echo a > a
$ echo A > A
$ hg add a
$ hg st
A a
? A
$ hg add --config ui.portablefilenames=abort A
abort: possible case-folding collision for A
[255]
$ hg st
A a
? A
$ hg add A
warning: possible case-folding collision for A
$ hg st
A A
A a
$ hg forget A
$ hg st
A a
? A
$ hg add --config ui.portablefilenames=no A
$ hg st
A A
A a
$ mkdir b
$ touch b/c b/D
$ hg add b
adding b/D
adding b/c
$ touch b/d b/C
$ hg add b/C
warning: possible case-folding collision for b/C
$ hg add b/d
warning: possible case-folding collision for b/d
$ touch b/a1 b/a2
$ hg add b
adding b/a1
adding b/a2
$ touch b/A2 b/a1.1
$ hg add b/a1.1 b/A2
warning: possible case-folding collision for b/A2
$ touch b/f b/F
$ hg add b/f b/F
warning: possible case-folding collision for b/f
$ touch g G
$ hg add g G
warning: possible case-folding collision for g
$ mkdir h H
$ touch h/x H/x
$ hg add h/x H/x
warning: possible case-folding collision for h/x
$ touch h/s H/s
$ hg add h/s
$ hg add H/s
warning: possible case-folding collision for H/s
case changing rename must not warn or abort
$ echo c > c
$ hg ci -qAmx
$ hg mv c C
$ cd ..