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.

src/nvim/log.c

@@ -152,7 +152,17 @@ bool logmsg(int log_level, const char *context, const char *func_name, int line_
 #ifdef EXITFREE
   // Logging after we've already started freeing all our memory will only cause
   // pain.  We need access to VV_PROGPATH, homedir, etc.
-  assert(!entered_free_all_mem);
+  if (entered_free_all_mem) {
+    fprintf(stderr, "FATAL: error in free_all_mem\n %s %s %d: ", context, func_name, line_num);
+    va_list args;
+    va_start(args, fmt);
+    vfprintf(stderr, fmt, args);
+    va_end(args);
+    if (eol) {
+      fprintf(stderr, "\n");
+    }
+    abort();
+  }
 #endif
 
   log_lock();