BUG: filestorage: fix invalid usage of repo.add

Semantics of repo.add were changed in 94490d, but call sites were not
changed.
BUG: filestorage/dvcs_wrapper: make it raise understandable errors on DVCS failures rather than returning garbage
BUG: filestorage/dvcs_wrapper: control VCS command environment

This ensures that the VCS command doesn't pick up e.g. user names from
the environment.

Set home directory, to avoid using invalid hgrc/gitconfig files.
BUG: fix DVCS commit command

The user name must always be provided, as we intentionally avoid
supplying environment to the VCS command.