Fixed usage messages in subcommands corrupted after several usages

spec/usage_spec.lua

@@ -103,6 +103,23 @@ describe("tests related to usage message generation", function()
       )
    end)
 
+   it("usage messages for commands are correct after several :prepare() invocations", function()
+      local parser = Parser "foo"
+         :add_help(false)
+      parser:flag "-q" "--quiet"
+      local run = parser:command "run"
+         :add_help(false)
+      run:option "--where"
+
+      parser:prepare()
+      parser:prepare()
+
+      assert.equal(
+         [=[Usage: foo run [--where <where>]]=],
+         run:prepare():get_usage()
+      )
+   end)
+
    describe("usage generation can be customized", function()
       it("uses message provided by user", function()
          local parser = Parser "foo"

src/argparse.lua

@@ -267,6 +267,8 @@ function Parser:command(...)
 end
 
 function Parser:prepare()
+   self._fullname = self._fullname or self._name
+
    if self._add_help and not self._help_option then
       self._help_option = self:flag(self._add_help)
          :action(function()
@@ -295,7 +297,7 @@ function Parser:prepare()
 
    for _, command in ipairs(self._commands) do
       command._target = command._target or command._name
-      command._name = self._name .. " " .. command._name
+      command._fullname = self._fullname .. " " .. command._name
    end
 
    return self
@@ -322,7 +324,7 @@ local usage_welcome = "Usage: "
 
 function Parser:get_usage()
    if not self._usage then
-      local lines = {usage_welcome .. self._name}
+      local lines = {usage_welcome .. self._fullname}
 
       local function add(s)
          if #lines[#lines]+1+#s <= max_usage_width then