Fix: Restore buffer local settings outside reviewer

[jakubbortlik]

This PR fixes an issue originally reported in #347. After a closer inspection it is a gitlab.nvim bug, as currently we are removing keymaps and resetting 'modifiable' only for the currently active buffer, but not for other buffers previously shown by Diffview.

This PR fixes the issue by setting and deleting keymaps when entering any window that contains a Diffview buffer or when switching to a Diffview buffer without changing the window (i.e., cursor is already in the reviewer).

Edit: Apart from this fix, this PR also limits refreshing diagnostics to when the reviewer is entered, as opposed to the previous way when diagnostics were refreshed every time the reviewer buffer was changed. This speeds up the file switching significantly (although this is practically only noticeable when holding down <tab> to cycle between the diffs quickly which I don't think is something people do frequently 😄).

[jakubbortlik]

@briandipalma in August you reported the issue that I'm fixing in this PR, could you please try out this branch and let us know if it fixes the issue for you?

[briandipalma]

I've just gone on holidays so I doubt I'll get the time to test this until after Christmas, I'll take a look in January.

[briandipalma]

I've tried to recreate the issue but I'm failing so maybe it's gone, it was random so who knows. What I'll do is keep an eye out for the issue and if I see it I'll try this branch out. Sorry about taking so long.

[jakubbortlik]

I've tried to recreate the issue but I'm failing so maybe it's gone, it was random so who knows. What I'll do is keep an eye out for the issue and if I see it I'll try this branch out. Sorry about taking so long.

Hi Brian, thanks for having a look at this. Here's how you can reproduce the bug:

1. Open a MR with multiple modified files
2. In the Diffview File Panel press <tab> several times (or lua require("diffview.actions").select_next_entry())
3. run :ls<cr>, you should see in the buffer list, that the previously visited buffers are nomodifiable
4. In the File Panel or in one of the reviewer windows press gf which should open the file in another tab, where the current buffer is now modifiable and the commenting keymaps are removed (:map c should not show something like v c *@<Lua 1782: ~/projects/gitlab.nvim/lua/gitlab/reviewer/init.lua:384>\n Create comment for selected text.
5. In the same tab (i.e., not the gitlab.nvim tab) open one of the other reviewed files (e.g., by running :bprev). This file will be nomodifiable and :map c will show that the commenting mappings are still there

In the jakubbortlik:fix-buffer-local-settings-outside-reviewer branch, this should be fixed, i.g., step 3 will still show that the buffer you've just cycled through with Diffview's <tab> keymap are still shown as nomodifiable, but when you do step 5, you'll see that upon visiting the buffer in another tab, the options and mappings are fixed.