Makes vundle-before-update tag vundle-<datetime>

This way we can have multiple backups.

autoload/vundle/installer.vim

@@ -407,7 +407,7 @@ func! s:make_sync_command(bang, bundle) abort
 
     let cmd_parts = [
                 \ 'cd '.vundle#installer#shellesc(a:bundle.path()),
-                \ 'git tag -f vundle-before-update',
+                \ 'git tag -f vundle-'.strftime('%Y%m%d-%H%M'),
                 \ 'git pull',
                 \ 'git submodule update --init --recursive',
                 \ ]

doc/vundle.txt

@@ -278,11 +278,11 @@ or >
   :PluginUpdate vim-surround vim-fugitive
 
 The command `git tag` will be used to tag the commit that was used before any 
-update (i.e. before `git pull` is run). The tag is named 
-`vundle-before-update`.  If a plugin breaks after update, go to its directory 
-and run
+update (i.e. before `git pull` is run). The tag is named `update-<strftime>` 
+(today's date and time).  If a plugin breaks after update, go to its 
+directory, check for the latest tag using `git tag` and run
 
-  `git checkout vundle-before-update && \`
+  `git checkout vundle-<somedatetime> && \`
   `git submodule update --init --recursive`
 
 Then mark it as 'pinned' in `.vimrc`.