Diffview support (#56)

This MR adds support for diffview as a reviewer. I'll update the README and some other checks for Delta in a followup MR.

lua/gitlab/reviewer/diffview.lua

@@ -0,0 +1,71 @@
+-- This Module contains all of the code specific to the Delta reviewer.
+local state    = require("gitlab.state")
+local u        = require("gitlab.utils")
+
+local M        = {
+    bufnr = nil,
+    tabnr = nil
+}
+
+-- Public Functions
+-- These functions are exposed externally and are used
+-- when the reviewer is consumed by other code. They must follow the specification
+-- outlined in the reviewer/init.lua file
+M.open         = function()
+    vim.api.nvim_command(string.format("DiffviewOpen %s", state.INFO.target_branch))
+    M.tabnr = vim.api.nvim_get_current_tabpage()
+end
+
+M.jump         = function(file_name, new_line, old_line)
+    if M.tabnr == nil then
+        vim.notify("Can't jump to Diffvew. Is it open?", vim.log.levels.ERROR)
+        return
+    end
+    vim.api.nvim_set_current_tabpage(M.tabnr)
+    vim.cmd("DiffviewFocusFiles")
+    local view = require("diffview.lib").get_current_view()
+    if view == nil then
+        vim.notify("Could not find Diffview view", vim.log.levels.ERROR)
+        return
+    end
+    local files = view.panel:ordered_file_list()
+    local layout = view.cur_layout
+    for _, file in ipairs(files) do
+        if file.path == file_name then
+            view:set_file(file)
+            if new_line ~= nil then
+                layout.b:focus()
+                vim.api.nvim_win_set_cursor(0, { tonumber(new_line), 0 })
+            elseif old_line ~= nil then
+                layout.a:focus()
+                vim.api.nvim_win_set_cursor(0, { tonumber(old_line), 0 })
+            end
+            break
+        end
+    end
+end
+
+M.get_location = function()
+    if M.tabnr == nil then return nil, nil, "Diffview reviewer must be initialized first" end
+    local bufnr = vim.api.nvim_get_current_buf()
+    -- check if we are in the diffview tab
+    local tabnr = vim.api.nvim_get_current_tabpage()
+    if tabnr ~= M.tabnr then return nil, nil, "Line location can only be determined within reviewer window" end
+    -- check if we are in the diffview buffer
+    local view = require("diffview.lib").get_current_view()
+    local layout = view.cur_layout
+    local file_name = nil
+    local current_line_changes = nil
+    if layout.a.file.bufnr == bufnr then
+        file_name = layout.a.file.path
+        current_line_changes = { new_line = nil, old_line = vim.api.nvim_win_get_cursor(0)[1] }
+        return file_name, current_line_changes
+    elseif layout.b.file.bufnr == bufnr then
+        file_name = layout.b.file.path
+        current_line_changes = { new_line = vim.api.nvim_win_get_cursor(0)[1], old_line = nil }
+        return file_name, current_line_changes
+    end
+    return nil, nil, "Line location can only be determined within reviewer window"
+end
+
+return M