cmd/cue: fix cue help cmd mycmd ./mypkg

Jonathan Matthews correctly pointed out that a tutorial used the command
successfully up to and including CUE v0.8.0, but it broke from v0.9.0.
My refactor at https://cuelang.org/cl/1194247 was overall good,
but it did break this edge case, which in my defense had no tests.

Add tests, and fix the code. The issue was that we left args as
["cmd", "mycmd", "./mypkg"], where the valid command is only
["cmd", "mycmd"], so we treated this scenario as an unknown subcommand.

Fixes #3462.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I2208fa8411f1122ede863b8dc6a5d81805562f30
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201709
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

cmd/cue/cmd/help.go

@@ -58,10 +58,12 @@ func newHelpCmd(c *Command) *cobra.Command {
 				//	["cmd", "mycmd"]
 				//	["cmd", "mycmd", "./mypkg"]
 				//
-				// We want to skip the first two arguments in pkgArgs.
-				pkgArgs := args[1:]
-				if len(pkgArgs) > 0 {
-					pkgArgs = pkgArgs[1:]
+				// In the third case, we want to load ["./mypkg"]
+				// and we want to look up the help for ["cmd", "mycmd"].
+				var pkgArgs []string
+				if len(args) > 2 {
+					pkgArgs = args[2:]
+					args = args[:2]
 				}
 
 				tools, err := buildTools(c, pkgArgs)

cmd/cue/cmd/testdata/script/help_cmd.txtar

@@ -8,6 +8,14 @@ cmp stdout cue-help-cmd.stdout
 exec cue help cmd hello
 cmp stdout cue-help-cmd-hello.stdout
 
+exec cue help cmd hello .
+cmp stdout cue-help-cmd-hello.stdout
+
+mkdir subdir
+cp task_tool.cue subdir
+exec cue help cmd hello ./subdir
+cmp stdout cue-help-cmd-hello.stdout
+
 ! exec cue help cmd missing
 stderr -count=1 'Unknown cmd command: missing'
 stderr -count=1 '^Available Commands:$'