win: vim_FullName(): force backslashes #7287

- Replace obvious cases of '/' literal with PATHSEP. (There are still
  some remaining cases that need closer inspection.)
- Fixup tests: ui/screen_basic

closes #7117
ref https://github.com/neovim/neovim/issues/2471#issuecomment-271193714

test/functional/core/path_spec.lua

@@ -0,0 +1,61 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local eval = helpers.eval
+local get_pathsep = helpers.get_pathsep
+local command = helpers.command
+
+describe("'%:p' expanding", function()
+  local pathsep
+  local targetdir
+  local expected_path
+
+  local function get_full_path()
+    return eval('expand("%:p")')
+  end
+
+  local function join_path(...)
+    return table.concat({...}, pathsep)
+  end
+
+  before_each(function()
+    clear()
+    pathsep = get_pathsep()
+    targetdir = join_path('test', 'functional', 'fixtures')
+    clear(join_path(targetdir, 'tty-test.c'))
+    expected_path = get_full_path()
+  end)
+
+  it('given a relative path with current directory in the middle #7117', function()
+    clear(join_path(targetdir, '.', 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with current directory #7117', function()
+    clear(join_path('.', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with current directory to a file when changing directory #7117', function()
+    clear(join_path('.', targetdir, 'tty-test.c'))
+    command('cd test')
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a relative path with directory up the tree to a file #7117', function()
+    clear(join_path(targetdir, '..', 'fixtures', 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a different starting directory and a relative path with directory up the tree #7117', function()
+    command('cd test')
+    command('e ' .. join_path('..', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+
+  it('given a different starting directory and a relative path with current directory and up the tree #7117', function()
+    command('cd test')
+    command('e ' .. join_path('.', '..', targetdir, 'tty-test.c'))
+    eq(expected_path, get_full_path())
+  end)
+end)