godoc: correct abspath when looking for cmds

godoc is erroneously detecting paths like syscall and unsafe as possible
commands.

A previous attempt at a fix adjusted the parameters of pkgHandler in
pres.go, but that change had unexpected ripple effects that broke links
and other features of godoc. This change is scoped only to the godoc
command line tool.

cmdline_test.go is updated to match the parameters used in the real
pkgHandler in pres.go.

Fixes golang/go#14447

Change-Id: I8f740c6847e46523b8443722b16942192bdf9cb8
GitHub-Last-Rev: cfc24f2d4c
GitHub-Pull-Request: golang/tools#27
Reviewed-on: https://go-review.googlesource.com/96515
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/cmdline.go

@@ -72,8 +72,8 @@ func CommandLine(w io.Writer, fs vfs.NameSpace, pres *Presentation, args []strin
 		abspath = pathpkg.Join(pres.PkgFSRoot(), toolsPath+path)
 		cinfo = pres.GetCmdPageInfo(abspath, relpath, mode)
 		if cinfo.IsEmpty() {
-			// Then try $GOROOT/cmd.
-			abspath = pathpkg.Join(pres.CmdFSRoot(), path)
+			// Then try $GOROOT/src/cmd.
+			abspath = pathpkg.Join(pres.CmdFSRoot(), cmdPrefix, path)
 			cinfo = pres.GetCmdPageInfo(abspath, relpath, mode)
 		}
 	}

godoc/cmdline_test.go

@@ -206,7 +206,7 @@ package main
 		p:       p,
 		c:       c,
 		pattern: "/cmd/",
-		fsRoot:  "/src/cmd",
+		fsRoot:  "/src",
 	}
 	p.pkgHandler = handlerServer{
 		p:       p,