Presumably the lines in question were changed in one branch or the other (not
necessarily by messing with white space), and our git settings are suggesting
that you omit the spaces before tabs while you're futzing with those lines.
As build-aux/git-hooks/pre-commit suggests, you can work around the problem
temporarily with the shell command 'git config core.whitespace -trailing-space'.
I don't recommend this setting in general, though.
I found the error useful. So when line-numbers branch is eventually merged, instead of ignoring/bypassing this error, I'd suggest that that tab+space+tab combo be fixed and that whole thing be made to contain only tabs or only spaces (my preference).
Slowly and gradually, as more merges and commits happen in future, all such mixtures of tabs+spaces will get removed.