cabalg is an alias for installing cabal package from a git source repository.
E.g.
$ git clone https://github.com/author/foo
$ git clone https://github.com/author/bar
$ cabal install foo/foo.cabal bar/bar.cabal baz.cabal
could be abbreviated by
$ cabalg https://github.com/author/foo https://github.com/author/bar -- baz.cabal
It also supports arbitrary git revisions mentioning like
$ cabalg https://github.com/baz/quux@f524f97
Necessary arguments could be passed to cabal install with -- delimiter like
$ cabalg <repo1> ... <repoN> [-- <cabal-install args>]
Please notice that --single-branch flag comes with git-1.7.10 and later, so you probably want to have it.
It's supposed to be Windows-compatible.