rust-cpython: add panicking version of borrow_mut() and use it
The original borrow_mut() is renamed to try_borrow_mut().
Since leak_immutable() no longer incref the borrow count, the caller should
know if the underlying value is borrowed or not. No Python world is involved.
That's why we can simply use the panicking borrow_mut().
$ hg init a
$ cd a
$ echo 123 > a
$ hg add a
$ hg commit -m "a" -u a
$ cd ..
$ hg init b
$ cd b
$ echo 321 > b
$ hg add b
$ hg commit -m "b" -u b
$ hg pull ../a
pulling from ../a
searching for changes
abort: repository is unrelated
[255]
$ hg pull -f ../a
pulling from ../a
searching for changes
warning: repository is unrelated
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
new changesets 9a79c33a9db3
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg heads
changeset: 1:9a79c33a9db3
tag: tip
parent: -1:000000000000
user: a
date: Thu Jan 01 00:00:00 1970 +0000
summary: a
changeset: 0:01f8062b2de5
user: b
date: Thu Jan 01 00:00:00 1970 +0000
summary: b
$ cd ..