Make foldmethod configurable or do not override it

Similar to commit 64619e4 ("Respect the user's foldlevelstart settings"), the user's foldmethod should not get overwritten to "syntax" probably.

It looks like there are two places where this gets done:

```
./plugin/gitv.vim: silent setlocal fdm=syntax
./plugin/gitv.vim: call s:MoveIntoPreviewAndExecute('setlocal fdm=syntax', 0)
```

This should be made either configurable, or documented how to accomplish this using autocommands (e.g. by matching for "fugitive://" buffers).

"foldmethod=syntax" causes the diff hunks to be closed, while I would like to have them open by default instead.