Fix tig-explorer issues

[zhimsel]

https://github.com/iberianpig/tig-explorer.vim

For some reason, tig-explorer has a hard time returning to the correct buffer when it takes over a window. When tig is closed, it often returns to the incorrect buffer.

It also is unable to deal with manually-set $GIT_DIR and $GIT_WORK_TREE env vars (when .git is not present).

I'll probably end up needing to rewrite a lot of the plugin.

To-do

• [ ] Use $TIG_SCRIPT instead of hacking a temporary tigrc file; alternatively, call a custom config file that sources the users'.
• [ ] Use -C option instead of having to temporarily lcd to the project root
• [ ] Figure out how to restore the proper buffer

[zhimsel]

Not going to bother, since I just use Tig directly in a Terminal window now. It's slightly clunky (due to the nested vim sessions if I edit a commit message), but it works well enough.