Add --relvsn and --relname to rebar3 shell

Allows to select one of many releases conditionally.
Default behaviour should be unchanged from existing one.

Can filter on only the version or only the name, or both.

src/rebar_prv_shell.erl

@@ -78,6 +78,12 @@ init(State) ->
                          "shell. (E.g. --apps app1,app2,app3) Defaults "
                          "to rebar.config {shell, [{apps, Apps}]} or "
                          "relx apps if not specified."},
+                        {relname, $r, "relname", atom,
+                         "Name of the release to use as a template for the "
+                         "shell session"},
+                        {relvsn, $v, "relvsn", string,
+                         "Version of the release to use for the shell "
+                         "session"},
                         {start_clean, undefined, "start-clean", boolean,
                          "Cancel any applications in the 'apps' list "
                          "or release."},
@@ -335,15 +341,30 @@ find_apps_rebar(State) ->
 
 -spec find_apps_relx(rebar_state:t()) -> no_value | list().
 find_apps_relx(State) ->
-    case lists:keyfind(release, 1, rebar_state:get(State, relx, [])) of
-        {_, _, Apps} ->
+    {Opts, _} = rebar_state:command_parsed_args(State),
+    RelxOpts = rebar_state:get(State, relx, []),
+    {Defname, Defvsn} = debug_get_value(default_release, RelxOpts,
+                                        {undefined, undefined},
+                                        "Found default release from config"),
+    Relname = debug_get_value(relname, Opts, Defname,
+                              "Found relname from command line option"),
+    Relvsn = debug_get_value(relvsn, Opts, Defvsn,
+                             "Found relvsn from command line option"),
+    Releases = [Rel || Rel <- rebar_state:get(State, relx, []),
+                       is_tuple(Rel), element(1, Rel) =:= release,
+                       tuple_size(Rel) =:= 3 orelse tuple_size(Rel) =:= 4,
+                       {Name, Vsn} <- [element(2, Rel)],
+                       Relname == undefined orelse Name == Relname,
+                       Relvsn == undefined orelse Vsn == Relvsn],
+    case Releases of
+        [] ->
+            no_value;
+        [{_, _, Apps}|_] ->
             ?DEBUG("Found shell apps from relx.", []),
             Apps;
-        {_, _, Apps, _} ->
+        [{_, _, Apps, _}|_] ->
             ?DEBUG("Found shell apps from relx.", []),
-            Apps;
-        false ->
-            no_value
+            Apps
     end.
 
 load_apps(Apps) ->