Use replace-buffer-contents to update formatting

[ljos]

blacken.el (blacken-buffer): Use replace-buffer-contents to update
formatting after running black. This is potentially slower than using
copy-to-buffer, but it keeps the position of point etc (if it can).

blacken.el (blacken-replace-buffer-contents-max-secs): Add new
variable to control how long to spend calculating
replac-buffer-contents.

Fixes #33
Fixes #15

[ljos]

I set the blacken-replace-buffer-contents-max-secs to nil as setting it can lead to unexpected behaviour if the timeout is reached sometimes, but not others. Then the buffer is replaced in its entirety and we lose position again.

The other issue is that replace-buffer-contents was introduced in emacs 26.1, so this change drops support for versions previous to that.

[dakra]

Personally I don't use blacken but replace-buffer-contents only got added Emacs 26 and the MAX-SEC parameter only Emacs 27.

I guess we could drop Emacs 25 support? But depending on Emacs 27 is surely too new.

Also I read about a couple of problems already with replace-buffer-contents.
https://github.com/purcell/emacs-reformatter/blob/b57f5d480003ab7b0880e0059dcc51747fb2e088/reformatter.el#L296

[ljos]

I wasn't aware of reformatter. I would suggest it is probably more robust using that. I see that there is already an alternative using it on GitHub: https://github.com/wbolster/emacs-python-black

[dakra]

Yes. I guess emacs-python-black is better maintained than this repository here (which is on life support only ;)

Personally I use https://github.com/raxod502/apheleia and are happy with it.