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/edit.c

@@ -3775,7 +3775,7 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
         // again when auto-formatting.
         if (has_format_option(FO_AUTO)
             && has_format_option(FO_WHITE_PAR)) {
-          char *ptr = ml_get_buf(curbuf, curwin->w_cursor.lnum, true);
+          char *ptr = ml_get_buf_mut(curbuf, curwin->w_cursor.lnum);
           int len;
 
           len = (int)strlen(ptr);