improve vanadium-go-generate output
improve vanadium-go-generate output

Output will now separate the erroneous files a little better. It will also
produce a diff showing what change go generate would make. This output is also
captured in the xunit report.

Here's an example:

---- EXAMPLE ----
The following go generated files are not up-to-date:
	* release/go/src/v.io/x/ref/cmd/vdl/doc.go

Diff for release/go/src/v.io/x/ref/cmd/vdl/doc.go:
diff --git a/cmd/vdl/doc.go b/cmd/vdl/doc.go
index 5cc386a..2844484 100644
--- a/cmd/vdl/doc.go
+++ b/cmd/vdl/doc.go
@@ -20,10 +20,10 @@ The vdl commands are:
    help        Display help for commands or topics

 The vdl additional help topics are:
-   packages    Description of package lists
-   vdlpath     Description of VDLPATH environment variable
-   vdlroot     Description of VDLROOT environment variable
-   vdl.config  Description of vdl.config files
+   packagesssss Description of package lists
+   vdlpath      Description of VDLPATH environment variable
+   vdlroot      Description of VDLROOT environment variable
+   vdl.config   Description of vdl.config files

 The vdl flags are:
  -exts=.vdl

Change-Id: I0c9359fa853f6673de7237cfb586e48dc1d0d52b

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools