'real_path' functions where not returning last part of path.

'readlink -e' is as non-portable as 'readlink -f' it would seem.

Simplified checkout process and now push ref as well as checkout by
commit ID.

[auto-build] log-location is now the config value for the base of the
logging tree, and the default is now /var/log/auto-build

Reversed the order of reference prefixes tried when matching a config
section from least significant first to most significant first.

Shell globing disabled around the retrieval of section names (which
are ref patterns) to prevent matching file paths.

This reverts commit 1600e768.

Config section names (which are ref patterns) quoted to prevent
matching file paths.

auto-build commands that use shell operations like '&&' need to
actually invoke the shell. The autoconf profile command now
invokes 'bash -c "... && ..."'

To avoid unexpected permissions on build files the build directories
must not have a sticky bit set. The bit is unset on directory creation.

To support relative links `readlink -f ...` was used on the build
command. The problem with this is if arguments are included in the
build command it breaks. The command line is now broken up and
`readline -e ...` run on the first part. (-e ensures that the target
exists.)