Use _git_inout for git.show

so we don't spew the error message on stdout for nonexistent objects
Add minimal vfs interface

so we can access blobs in git as file like objects
Introduce Source class

so we don't have to expose all the details of Debian's different
files and conventions.
Honor debian/source/format

when checking if a package is a native package

Closes: #669267
Add gbp.git.source.Source.sourcepkg

to easily fetch the source package name

Git-Dch: Ignore
Document changes and release 0.6.0~git20130413
Don't try access source before it's filled
Document changes and release 0.6.0~git20130414