Port capture() function

https://groups.google.com/forum/#!msg/vim_dev/H3Z3ChSUh_4/beZs6KzYdBsJ

test/functional/eval/capture_spec.lua

@@ -0,0 +1,86 @@
+local helpers = require('test.functional.helpers')
+local eq = helpers.eq
+local eval = helpers.eval
+local clear = helpers.clear
+local source = helpers.source
+local redir_exec = helpers.redir_exec
+local exc_exec = helpers.exc_exec
+local funcs = helpers.funcs
+local Screen = require('test.functional.ui.screen')
+local feed = helpers.feed
+
+describe('capture()', function()
+  before_each(clear)
+
+  it('returns the same result with :redir', function()
+    eq(redir_exec('messages'), funcs.capture('messages'))
+  end)
+
+  it('returns the output of the commands if the argument is List', function()
+    eq("foobar", funcs.capture({'echon "foo"', 'echon "bar"'}))
+    eq("\nfoo\nbar", funcs.capture({'echo "foo"', 'echo "bar"'}))
+  end)
+
+  it('supports the nested redirection', function()
+    source([[
+    function! g:Foo()
+      let a = ''
+      redir => a
+      silent echon "foo"
+      redir END
+      return a
+    endfunction
+    function! g:Bar()
+      let a = ''
+      redir => a
+      call g:Foo()
+      redir END
+      return a
+    endfunction
+    ]])
+    eq('foo', funcs.capture('call g:Bar()'))
+
+    eq('42', funcs.capture([[echon capture("echon capture('echon 42')")]]))
+  end)
+
+  it('returns the transformed string', function()
+    eq('^A', funcs.capture('echon "\\<C-a>"'))
+  end)
+
+  it('returns the empty string if the argument list is empty', function()
+    eq('', funcs.capture({}))
+    eq(0, exc_exec('let g:ret = capture(v:_null_list)'))
+    eq('', eval('g:ret'))
+  end)
+
+  it('returns the errors', function()
+    local ret
+    ret = exc_exec('call capture(0.0)')
+    eq('Vim(call):E806: using Float as a String', ret)
+    ret = exc_exec('call capture(v:_null_dict)')
+    eq('Vim(call):E731: using Dictionary as a String', ret)
+    ret = exc_exec('call capture(function("tr"))')
+    eq('Vim(call):E729: using Funcref as a String', ret)
+    ret = exc_exec('call capture(["echo 42", 0.0, "echo 44"])')
+    eq('Vim(call):E806: using Float as a String', ret)
+    ret = exc_exec('call capture(["echo 42", v:_null_dict, "echo 44"])')
+    eq('Vim(call):E731: using Dictionary as a String', ret)
+    ret = exc_exec('call capture(["echo 42", function("tr"), "echo 44"])')
+    eq('Vim(call):E729: using Funcref as a String', ret)
+  end)
+
+  it('silences command run inside', function()
+    local screen = Screen.new(20, 5)
+    screen:attach()
+    screen:set_default_attr_ignore({{bold=true, foreground=255}})
+    feed(':let g:mes = capture("echon 42")<CR>')
+    screen:expect([[
+    ^                    |
+    ~                   |
+    ~                   |
+    ~                   |
+                        |
+    ]])
+    eq('42', eval('g:mes'))
+  end)
+end)