This patch adds support for customising the way in which rebar generates
version numbers for app.src files using the `{vsn,Spec}` approach.
Whilst the existing `{vsn,ScmName::atom()}` syntax will continue to
work, users can also pass `{vsn,{cmd,Cmd::string()}}` in which
case the provided *command* will be used. For example:

```erlang
{application, doodah,
 [
  {vsn, {cmd, "git rev-parse --short HEAD"}}]}.
```

Allows using tools like git-subtree and still maintaining proper
git-based vsns for those "imported" subtrees. Also helps understanding
what was the last change introduced to a specific application within a
multi-application repository (at this moment, all applications that
reside in the same repository, will share the same autogenerated git
vsn)

Replace rebar_util:get_cwd/0 by rebar_utils:get_cwd/0.  Luckily the
misspelt module name doesn't seem to have caused any harm, since
rebar_app_utils:is_app_dir/0 and rebar_rel_utils:is_rel_dir/0 aren't
called (only their /1 counterparts).

--HG--
extra : rebase_source : 05a045be5a