update .gitignore: track cobra subdirectory (spf13#936)

The 'cobra' entry on .gitignore was meant to ignore a potential 'cobra'
executable generated by 'go build' during development. However, 'cobra'
is a directory containing the command line tool, and such a generic
entry in .gitignore ignores too much.

Instead of the generic 'cobra' entry, ignore only a specific
'/cobra/cobra' path.

Author: rhcarvalho

.gitignore

@@ -32,6 +32,7 @@ Session.vim
 tags
 
 *.exe
+/cobra/cobra
 cobra.test
 bin