Problem:
When ui2 is enabled, opening an already-open file in another Neovim
instance results in the error `E812: Autocommands changed buffer…`.
Analysis:
On E812 the file is not loaded. The default SwapExists handler notifies
`W325: Ignoring swapfile…`, and ui2 shows that message by opening
a window, which is a temp context switch. `ctx_dirs_restore()`
re-shortens every buffer name on the way out, and `shorten_buf_fname()`
always frees and reallocates `b_sfname`. `readfile()` aliases `b_fname`
across `check_need_swap()` and compares the pointer to detect a rename.
Regression by b296666e41, which replaced the `cs_save_sfname` restore
(that kept curbuf's pointer) with `shorten_fnames(true)`.
Solution:
Keep the allocation in `shorten_buf_fname()` when the short name is
unchanged. Pointer stability is what the E200/E201/E812 guards actually
assert.
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