fix: don't call load_oil_buffer in open_preview if oil_ready is not set

[FelipeSharkao]

Based on the fix in barrettruth#12.

In their words:

When the preview window opens a directory that already has a loaded oil buffer with unsaved edits, open_preview() unconditionally calls load_oil_buffer() on it. This re-initializes the buffer via view.initialize() → render_buffer_async(), which re-fetches the directory listing from disk and replaces all buffer lines, destroying the user's pending edits (e.g. a cut line). The mutation parser then can't see the deleted entry in the source buffer, producing a COPY action instead of a MOVE.

[...]

Guard the load_oil_buffer() call in open_preview() with a check for vim.b[filebufnr].oil_ready. Buffers that are already initialized and rendered skip re-loading, preserving any unsaved modifications.

I verified the fix manually, though I did not do an in-depth analysis to see if this breaks anything else.

Closes #632