fix: preserve the alternate buffer (#20)

tests/altbuf_spec.lua

@@ -0,0 +1,49 @@
+local oil = require("oil")
+local test_util = require("tests.test_util")
+local fs = require("oil.fs")
+
+describe("Alternate buffer", function()
+  after_each(function()
+    test_util.reset_editor()
+  end)
+
+  it("sets previous buffer as alternate", function()
+    vim.cmd.edit({ args = { "foo" } })
+    oil.open()
+    vim.cmd.edit({ args = { "bar" } })
+    assert.equals("foo", vim.fn.expand("#"))
+  end)
+
+  it("sets previous buffer as alternate when editing oil://", function()
+    vim.cmd.edit({ args = { "foo" } })
+    vim.cmd.edit({ args = { "oil://" .. fs.os_to_posix_path(vim.fn.getcwd()) } })
+    vim.cmd.edit({ args = { "bar" } })
+    assert.equals("foo", vim.fn.expand("#"))
+  end)
+
+  it("preserves alternate buffer if editing the same file", function()
+    vim.cmd.edit({ args = { "foo" } })
+    vim.cmd.edit({ args = { "bar" } })
+    oil.open()
+    vim.cmd.edit({ args = { "bar" } })
+    assert.equals("foo", vim.fn.expand("#"))
+  end)
+
+  it("preserves alternate buffer if discarding changes", function()
+    vim.cmd.edit({ args = { "foo" } })
+    vim.cmd.edit({ args = { "bar" } })
+    oil.open()
+    oil.close()
+    assert.equals("foo", vim.fn.expand("#"))
+  end)
+
+  it("sets previous buffer as alternate after multi-dir hops", function()
+    vim.cmd.edit({ args = { "foo" } })
+    oil.open()
+    oil.open()
+    oil.open()
+    oil.open()
+    vim.cmd.edit({ args = { "bar" } })
+    assert.equals("foo", vim.fn.expand("#"))
+  end)
+end)