fix: crash on case-only filename renames (case-insensitive filesystems)

[theAnuragMishra]

Renaming a file via oil where only the case changes (test.java ->
Test.java) throws invalid buffer error on a case-insensitive filesystem, but only if the file was already
open in a buffer.

Cause:
vim.fn.bufadd(dest_buf_name) resolves to the same buffer as
src_bufnr when the two names differ only in case, since they point
to the same inode. rename_buffer's swap logic assumes src_bufnr
and dest_bufnr are always distinct, so it deletes src_bufnr and
then accesses dest_bufnr (the now-deleted same buffer) right after.

Fix:
Short-circuit when dest_bufnr == src_bufnr: just rename the buffer
in place and clean up the old-name buffer.

fixes #778

[theAnuragMishra]

@stevearc can you check this out?