I doubt if the ability to work on unzipped content is ever going to be an integral part of Sigil, quite frankly. It's an epub editor--and epubs are zipped. There are plenty of other editors that will easily allow anyone to manipulate unzipped html/css/xml source files.
It probably wouldn't be all that difficult, however, for someone to create a Sigil input plugin that could be used to "pull" a git/svn repository containing a versioned OEBPS structure. It could even be used in conjunction with a complimentary output plugin that pushed changed files back to a repository.
It would probably never be perfect, however, since Sigil/Gumbo might rearrange attribute orders in otherwise "unchanged" files, but it's probably doable.
EDIT: I see KevinH came up with a similar suggestion while I was typing.
