git.py: don't automatically initialize ~/.bup if it doesn't exist.

bup had a convenience feature where commands would automagically
initialize a repo in ~/.bup if it didn't exist and no other BUP_DIR
was given.

This had the odd effect that when one forgot to specify BUP_DIR, a bup
repo would be initialized in ~/.bup even though only a browsing
command was used.

This patch drops that behaviour.  Now all repositories must be
explicitly intiialized via "bup init".

Signed-off-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: edit commit message; fix test ("set +e" during
 init run).]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
README.md: add a "bup init" step to "Using bup" since we killed auto-init.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>