git-fetch ansi control sequences

Running 'git fetch' in a dumb terminal (like an Emacs shell buffer)
now outputs raw control sequences, which isn't particularly pretty:

| remote: Generating pack...^[[K
| remote: Done counting 4044 objects.^[[K
| remote: Result has 2577 objects.^[[K
| remote: Deltifying 2577 objects...^[[K
| remote:
| remote: Total 2577 (delta 2150), reused 2457 (delta 2044)^[[K

It would be easy to make recv_sideband() check if the terminal is
dumb, but I have a feeling that the sideband code shouldn't have to
know about such things, being rather generic.

What do people think?