feat: cycle previewers with commit and bcommit already using it (#528)

- new git previewers
- jump to line in bcommit previewer
- vimdiff for bcommits
- dynamic preview window titles
- more previewers documentation

Cycle previewers are not mapped yet. So you need to setup yourself:
```lua
require('telescope').setup {
  defaults = {
    mappings = {
      i = {
        ["<C-s>"] = actions.cycle_previewers_next,
        ["<C-a>"] = actions.cycle_previewers_prev,
      },
    },
  }
}
```

Co-authored-by: Thore Strassburg <thore@weilbier.net>

lua/telescope/config.lua

@@ -147,6 +147,14 @@ function config.set_defaults(defaults)
 
   set("file_ignore_patterns", nil)
 
+  set("dynamic_preview_title", false, [[
+    Will change the title of the preview window dynamically, where it
+    is supported. Means the preview window will for example show the
+    full filename.
+
+    Default: false
+  ]])
+
   set("file_previewer", function(...) return require('telescope.previewers').vim_buffer_cat.new(...) end)
   set("grep_previewer", function(...) return require('telescope.previewers').vim_buffer_vimgrep.new(...) end)
   set("qflist_previewer", function(...) return require('telescope.previewers').vim_buffer_qflist.new(...) end)