Decide which version you want.
master branch is where development happens;
HEAD is usually OK, including the latest fixes and portability tweaks,
but an occasional regression may happen despite our (limited) test suite.
release branch is what cautious people should be using;
it has usually been tested more, and releases are cut at a point
where there isn't any known unresolved issue.
You may get the ASDF source repository using git: git clone git://common-lisp.net/projects/asdf/asdf.git
You will find the above referenced tags in this repository. You can also browse the repository on http://common-lisp.net/gitweb?p=projects/asdf/asdf.git.
Discussion of ASDF development is conducted on the mailing list firstname.lastname@example.org. http://common-lisp.net/cgi-bin/mailman/listinfo/asdf-devel