fix: spurious exits from faulty :wq detection (#221)

The previous mechanism used histget() to get the last command or
expression to detect if the user issued a `:wq` or similar. This had the
issue where if a user issued a `:wq`, started vim again, then entered
oil and saved via some mechanism that is _not_ a command (e.g. a
keymap), we would incorrectly detect that `:wq` and exit after saving.
The new mechanism tracks all keypresses and may end up with false
negatives (e.g. ":wqaff<backspace><backspace>ll"), but those are less
frustrating than false positives.

lua/oil/ringbuf.lua

@@ -0,0 +1,37 @@
+---@class oil.Ringbuf
+---@field private size integer
+---@field private tail integer
+---@field private buf string[]
+local Ringbuf = {}
+function Ringbuf.new(size)
+  local self = setmetatable({
+    size = size,
+    buf = {},
+    tail = 0,
+  }, { __index = Ringbuf })
+  return self
+end
+
+---@param val string
+function Ringbuf:push(val)
+  self.tail = self.tail + 1
+  if self.tail > self.size then
+    self.tail = 1
+  end
+  self.buf[self.tail] = val
+end
+
+---@return string
+function Ringbuf:as_str()
+  local postfix = ""
+  for i = 1, self.tail, 1 do
+    postfix = postfix .. self.buf[i]
+  end
+  local prefix = ""
+  for i = self.tail + 1, #self.buf, 1 do
+    prefix = prefix .. self.buf[i]
+  end
+  return prefix .. postfix
+end
+
+return Ringbuf