main.c: "BufReadCmd term://": Skip existing terminal.

Check `exists('b:term_title')` to avoid the BufReadCmd for already-initialized
:terminal buffers.

Move the test for `:argadd`.
Add a test for `:edit<CR>`.
Tweak comments and code style.

test/functional/ex_cmds/edit_spec.lua

@@ -0,0 +1,26 @@
+local helpers = require("test.functional.helpers")(after_each)
+local eq, execute, funcs = helpers.eq, helpers.execute, helpers.funcs
+local ok = helpers.ok
+local clear = helpers.clear
+
+describe(":edit", function()
+  before_each(function() 
+    clear()
+  end)
+
+  it("without arguments does not restart :terminal buffer", function()
+      execute("terminal")
+      helpers.feed([[<C-\><C-N>]])
+      local bufname_before = funcs.bufname("%")
+      local bufnr_before = funcs.bufnr("%")
+      helpers.ok(nil ~= string.find(bufname_before, "^term://"))  -- sanity
+
+      execute("edit")
+
+      local bufname_after = funcs.bufname("%")
+      local bufnr_after = funcs.bufnr("%")
+      ok(funcs.line('$') > 1)
+      eq(bufname_before, bufname_after)
+      eq(bufnr_before, bufnr_after)
+  end)
+end)