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/bufwrite.c

@@ -1524,7 +1524,7 @@ restore_backup:
     for (lnum = start; lnum <= end; lnum++) {
       // The next while loop is done once for each character written.
       // Keep it fast!
-      char *ptr = ml_get_buf(buf, lnum, false) - 1;
+      char *ptr = ml_get_buf(buf, lnum) - 1;
       if (write_undo_file) {
         sha256_update(&sha_ctx, (uint8_t *)ptr + 1, (uint32_t)(strlen(ptr + 1) + 1));
       }