bashrc: use PATH_* for directory path vars

My previous convention was *_DIR, but that sometimes clashes with
internal conventions of various projects. For example, GIT_DIR is used
by git to mean something special, so we do not want to set that var.

Use of PATH_* seems much less likely to clash with any existing
convention, and has the added bonus that all my defined directory vars
will now be grouped together by env, etc.
bashrc: fix SVN_AUTHORS definition

It needs to come after the definition of directory paths.
It also needs to point to CFG_PRIVATE, not CONFIG.
bashrc: add git repository shortcuts