Use correct ERL_LIBS separator on Windows.

This change adds the ability to use alternate urls for downloading
dependencies. To make use of alternate urls run:
rebar get-deps alt_urls=true

This patch modifies rebar_core to allow plugins to participate in the
pre and post processing steps, giving plugin authors more flexibility
and control.

Export two extra environment variables when executing shell commands.
These variables are useful for rebar hooks that rely on Erlang
applications installed as rebar dependencies.

$REBAR_DEPS_DIR contains a fully-qualified name of the directory where
rebar stores dependencies.

$ERL_LIBS is set to $REBAR_DEPS_DIR or to "$REBAR_DEPS_DIR:$ERL_LIBS",
if $ERL_LIBS was defined before.

In git origin/HEAD is a pointer to the default branch.  This patch
allows two alternatives to explicitly specifying "HEAD" in git VC specs.

The first is a 2 arity form {git, Url} and the second is {git, Url, ""}
which worked in pre-update-deps rebars.

Add flags to rebar_utils:sh to control output and error handling.
Replace calls to os:cmd with calls to rebar_utils:sh.

Change git downloads and updates to not create branches and fix git
tag handling.

The previous fix to relax the regex was insufficient.
This is basically the diff proposed by Bryan Fink with
the difference of using 'C' instead of 'en_US'.

Reported-by: Manuel Duran Aguete <manuel@aguete.org>

*If* there is a deps_dir tuple in the root rebar.config then
that is used globally as the deps dir. This is to stop dependencies for dependencies being
created in a different deps_dir even if the sub dependency so specifies.

--HG--
extra : rebase_source : bd95812644

--HG--
extra : rebase_source : 41c7f1c337