refactor(memline): distinguish mutating uses of ml_get_buf()

ml_get_buf() takes a third parameters to indicate whether the
caller wants to mutate the memline data in place. However
the vast majority of the call sites is using this function
just to specify a buffer but without any mutation. This makes
it harder to grep for the places which actually perform mutation.

Solution: Remove the bool param from ml_get_buf(). it now works
like ml_get() except for a non-current buffer. Add a new
ml_get_buf_mut() function for the mutating use-case, which can
be grepped along with the other ml_replace() etc functions which
can modify the memline.

src/nvim/buffer.h

@@ -138,7 +138,7 @@ static inline void buf_inc_changedtick(buf_T *const buf)
 static inline bool buf_is_empty(buf_T *buf)
 {
   return buf->b_ml.ml_line_count == 1
-         && *ml_get_buf(buf, (linenr_T)1, false) == '\0';
+         && *ml_get_buf(buf, (linenr_T)1) == '\0';
 }
 
 #endif  // NVIM_BUFFER_H