find_git_root doesn't work for submodules

[gitbugr]

Environment (please complete the following information):

• OS: Mac OS 10.15.1
• (Neo)Vim version: NVIM v0.5.0-nightly-851-g6e8c5779c
• vim-clap version: a2ee49e
• Have you reproduced with a minimal vimrc: n/a

Describe the bug
for git submodules, the .git/ folder is actually a file.

To Reproduce
Steps to reproduce the behavior:

1. Use any vim-clap configuration.

2. Create a git project git --init

3. Pull in your git submodule git submodule add https://github.com/liuchengxu/vim-clap

4. Pull in another git submodule git submodule add https://github.com/liuchengxu/vim-which-key

5. cd into the submodule you want to work on cd vim-clap

6. Open vim and search for a file Clap files - you will notice files from your other submodule

7. Check the git root echo clap#util#find_git_root(g:clap.start.bufnr) - the root is the parent repo. This is wrong.

Expected behavior
The git root should point to your submodule.

To fix this temporarily, I replaced the usages of clap#util#find_git_root(g:clap.start.bufnr) in autoload/clap/rooter.vim with the old clap#util#get_git_root()

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[liuchengxu]

Seemingly it's not easy to find the git root manually, even though that could be much faster than using the system call. If so, I think we should just stick to clap#util#get_git_root(), which takes about 200ms on my machine, I can live with that.

[liuchengxu]

Please see this branch fixes this issue, @gitbugr . I borrowed the strategy of vim-rooter, hoping that's robust enough, at least for the submodule case :(.