Made a significant change today: Enabled automatic retrying of transfers
that fail. It's only done if the previous try managed to advance the
progress by some amount. The assistant has already had that retrying for
many years, but now it will also be done when using git-annex at the
command line.

One good reason for a transfer to fail and need a retry is when the network
connection stalls. You'd think that TCP keepalives would detect this kind
of thing and kill the connection but I've had enough complaints, that I
suppose that doesn't always work or gets disabled. Ssh has a
ServerAliveInterval that detects such stalls nicely for the kind of batch
transfers git-annex uses ssh for, but it's not enabled by default. So I
found a way to make git-annex enable it, while still letting ~/.ssh/config
settings override that.

Also got back to analizing an old bug report about proliferating
".nfs*.lock" files when using git-annex on nfs; this was caused by the
wacky NFS behavior of renaming deleted files, and I found a change to the
ssh connection caching cleanup code that should avoid the problem.
