git.py: fix minor typos in comments

Signed-off-by: Holger Dell <github@holgerdell.com>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
mangle_path: verify chunked name refers to git dir

Before returning the mangled name.bup for a chunked item, ensure that
the name really does refer to a dir in the git tree via asssert.

Signed-off-by: Holger Dell <github@holgerdell.com>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>