Autocomplete all selections with <C-n>

[sauterp]

fixes #949

Autocomplete in INSERT mode by completes all selections based on
the suggestions generated by the input at the primary selection's cursor.

[Shugyousha]

Refactoring out the common parts like this may be nicer:

diff --git a/lua/plugins/complete-word.lua b/lua/plugins/complete-word.lua
index f0a6ff5..737cabe 100644
--- a/lua/plugins/complete-word.lua
+++ b/lua/plugins/complete-word.lua
@@ -2,17 +2,25 @@
 -- all non-primary selections will be completed aswell if their cursor
 -- is located at the end of the same word
 
+local get_prefix = function (file, pos)
+		if not pos then return "", false end
+
+		local range = file:text_object_word(pos > 0 and pos-1 or pos);
+		if not range then return "", false end
+		if range.finish > pos then range.finish = pos end
+		if range.start == range.finish then return "", false end
+		local prefix = file:content(range)
+		if not prefix then return "", false end
+		return prefix, true
+end
+
 vis:map(vis.modes.INSERT, "<C-n>", function()
 	local win = vis.win
 	local file = win.file
 	local pos = win.selection.pos
-	if not pos then return end
-	local range = file:text_object_word(pos > 0 and pos-1 or pos);
-	if not range then return end
-	if range.finish > pos then range.finish = pos end
-	if range.start == range.finish then return end
-	local prefix = file:content(range)
-	if not prefix then return end
+	prefix, ok = get_prefix(file, pos)
+	if not ok then return end
+
 	local cmd = string.format("vis-complete --word '%s'", prefix:gsub("'", "'\\''"))
 	local status, out, err = vis:pipe(file, { start = 0, finish = file.size }, cmd)
 	if status ~= 0 or not out then
@@ -21,14 +29,8 @@ vis:map(vis.modes.INSERT, "<C-n>", function()
 	end
 	for selection in win:selections_iterator() do
 		local pos = selection.pos
-		if not pos then goto continue end
-
-		local range = file:text_object_word(pos > 0 and pos-1 or pos);
-		if not range then goto continue end
-		if range.finish > pos then range.finish = pos end
-		if range.start == range.finish then goto continue end
-		local localprefix = file:content(range)
-		if not localprefix then goto continue end
+		localprefix, ok = get_prefix(file, pos)
+		if not ok then goto continue end
 		if localprefix ~= prefix then goto continue end
 
 		file:insert(pos, out)

Other than that it looks good to me (with very basic testing)!

[Shugyousha]

I hope the way to define the helper function is somewhat Lua-style conform (I am not very familiar with the language). This change looks good to me in any case!