Support 2 forms of implicit HEAD for git

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.

src/rebar_deps.erl

@@ -303,6 +303,10 @@ download_source(AppDir, {hg, Url, Rev}) ->
     rebar_utils:sh(?FMT("hg clone -U ~s ~s", [Url, filename:basename(AppDir)]),
                    [{cd, filename:dirname(AppDir)}]),
     rebar_utils:sh(?FMT("hg update ~s", [Rev]), [{cd, AppDir}]);
+download_source(AppDir, {git, Url}) ->
+    download_source(AppDir, {git, Url, "HEAD"});
+download_source(AppDir, {git, Url, ""}) ->
+    download_source(AppDir, {git, Url, "HEAD"});
 download_source(AppDir, {git, Url, {branch, Branch}}) ->
     ok = filelib:ensure_dir(AppDir),
     rebar_utils:sh(?FMT("git clone -n ~s ~s", [Url, filename:basename(AppDir)]),
@@ -344,6 +348,10 @@ update_source(Dep) ->
             Dep
     end.
 
+update_source(AppDir, {git, Url}) ->
+    update_source(AppDir, {git, Url, "HEAD"});
+update_source(AppDir, {git, Url, ""}) ->
+    update_source(AppDir, {git, Url, "HEAD"});
 update_source(AppDir, {git, _Url, {branch, Branch}}) ->
     ShOpts = [{cd, AppDir}],
     rebar_utils:sh("git fetch origin", ShOpts),