refactor(change): do API changes to buffer without curbuf switch

Most of the messy things when changing a non-current buffer is
not about the buffer, it is about windows. In particular, it is about
`curwin`.

When editing a non-current buffer which is displayed in some other
window in the current tabpage, one such window will be "borrowed" as the
curwin. But this means if two or more non-current windows displayed the buffers,
one of them will be treated differenty. this is not desirable.

In particular, with nvim_buf_set_text, cursor _column_ position was only
corrected for one single window. Two new tests are added: the test
with just one non-current window passes, but the one with two didn't.

Two corresponding such tests were also added for nvim_buf_set_lines.
This already worked correctly on master, but make sure this is
well-tested for future refactors.

Also, nvim_create_buf no longer invokes autocmds just because you happened
to use `scratch=true`. No option value was changed, therefore OptionSet
must not be fired.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 1cbfcff9f4..5d7ddb657a 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -1784,7 +1784,7 @@ failed:
       curbuf->b_p_ro = true;
     }
 
-    u_clearline();          // cannot use "U" command after adding lines
+    u_clearline(curbuf);   // cannot use "U" command after adding lines
 
     // In Ex mode: cursor at last new line.
     // Otherwise: cursor at first new line.
@@ -1793,7 +1793,7 @@ failed:
     } else {
       curwin->w_cursor.lnum = from + 1;
     }
-    check_cursor_lnum();
+    check_cursor_lnum(curwin);
     beginline(BL_WHITE | BL_FIX);           // on first non-blank
 
     if ((cmdmod.cmod_flags & CMOD_LOCKMARKS) == 0) {