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/api/private/helpers.c

@@ -517,7 +517,7 @@ String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col
     return rv;
   }
 
-  char *bufstr = ml_get_buf(buf, (linenr_T)lnum, false);
+  char *bufstr = ml_get_buf(buf, (linenr_T)lnum);
   size_t line_length = strlen(bufstr);
 
   start_col = start_col < 0 ? (int64_t)line_length + start_col + 1 : start_col;