Clear all old signs before reapplying

The previous implementation worked under the assumption that applying a
sign would overwrite any existing signs in the signcol. After reading :h
signcolumn it was clear that they were just getting added to a list and
the top priority sign was being rendered.

If a user had configured a signcolumn with more than 1 col then they
would see duplicate Hunk signs.

This fixes the issue by clearing all Hunk signs on a buffer before
reapplying new ones.

Fixes #2

lua/hunk/api/signs.lua

@@ -17,6 +17,12 @@ local M = {
   },
 }
 
+function M.clear_signs(buf)
+  vim.fn.sign_unplace("Hunk", {
+    buffer = buf,
+  })
+end
+
 function M.place_sign(buf, sign, linenr)
   vim.fn.sign_place(0, "Hunk", sign.name, buf, {
     lnum = linenr,

lua/hunk/ui/file.lua

@@ -66,6 +66,8 @@ function M.create(window, params)
   end
 
   local function apply_signs()
+    api.signs.clear_signs(buf)
+
     for _, hunk in ipairs(params.change.hunks) do
       for i in utils.hunk_lines(hunk[params.side]) do
         local is_selected = params.change.selected_lines[params.side][i]

lua/hunk/ui/tree.lua

@@ -218,6 +218,7 @@ function M.create(opts)
 
   function Component.render()
     tree:render()
+    signs.clear_signs(buf)
     apply_signs(tree, buf)
   end