fixes bug

I fixes following error, Please merge if it is good.

```
Error detected while processing /Users/konnok/repos/vim-signify/autoload/sy/repo.vim:
line   45:
E127: Cannot redefine function sy#repo#detect: It is in use
Error detected while processing function sy#start..sy#repo#detect:
line    2:
E117: Unknown function: sy#repo#get_diff_rcsdiff
E15: Invalid expression: sy#repo#get_diff_{type}(a:path)
Error detected while processing function sy#start:
line   15:
E714: List required
```