Fix processing of autoTagsDir option

[artem-kamyshev]

Hello, craigemery

First of all, thanks for the plugin.

This commit fixes two issues with g:autotagTagsDir.

1. I use ctags with fugurive plugin, so I place tags file into .git subdirectory. I've found out that though that this functionality is available in your plugin (via g:autotagTagsDir), it does not work. Here is a log, showing the reason (look at ..tst.cpp filename):

INFO:autotag_debug:source = "/home/kamyshev/workspace/personal/test-repo/tst.cpp"
INFO:autotag_debug:drive = "", file = "/home/kamyshev/workspace/personal/test-repo"
INFO:autotag_debug:testing tags_file "/home/kamyshev/workspace/personal/test-repo/.git/tags"
INFO:autotag_debug:Stripping tags for ..tst.cpp from tags file /home/kamyshev/workspace/personal/test-repo/.git/tags

2. With this fixed, I've found out that it cannot open tags file if cwd in vim is not a root project directory.

Here is what I mean. Test project hierarchy is as following:

<rootdir>/.git/tags
<rootdir>/tst.cpp
<rootdir>/d1/x.h

When we call vim from <rootdir> everything is fine. When we call vim from <rootdir>/d1 (like this: cd <rootdir>/d1 && vim ./x.h), we get the following error:

WARNING:root:Traceback (most recent call last):
  File "/home/kamyshev/.vim/bundle/vim-autotag/plugin/autotag.py", line 270, in autotag
    at.rebuildTagFiles()
  File "/home/kamyshev/.vim/bundle/vim-autotag/plugin/autotag.py", line 261, in rebuildTagFiles
    self.updateTagsFile(tags_dir, tags_file, sources)
  File "/home/kamyshev/.vim/bundle/vim-autotag/plugin/autotag.py", line 255, in updateTagsFile
    for l in do_cmd(cmd, self.tags_dir or tags_dir):
  File "/home/kamyshev/.vim/bundle/vim-autotag/plugin/autotag.py", line 63, in do_cmd
    p = subprocess.Popen(cmd, cwd=cwd, **kw)
  File "/usr/lib/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1343, in _execute_child
    raise child_exception
OSError: [Errno 2] Нет такого файла или каталога: '.git'

The reason is that though self.tags_dir does not contain a full path to tags file this value is used
as cwd argument when calling ctags application.

[craigemery]

I thought the parents = os.par * dir_count was to make a list of ".." that is then given to os.path.join() and os.path.join() puts in the os.sep bits. So I'm confused by this. I may try & replicate it with the new master & have a better look at the error.