GitRepository: make __build_env a static method

since cls is unused and it silences pychecker
Silence git clone

by using __git_inout instead of GitCommand
Silence _git_command

by using _git_inout instead of GitCommand
bash completion: handle upstream-tree like upstream-branch

That's as close as it gets since every ref like is valid
DebianSource: raise DebianSourceError if reading changelog fails

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
buildpackage: catch and handle DebianSourceError gracefully

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
tests: fix for older dch

Prevent dch from asking user input during tests.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
tests: another fix for older dch

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
GitRepository: add describe() method

Provides more output options than find_tag().
- longfmt: for getting the tag name in the long format (tag, number of
  commits and sha1)
- always: for falling back to sha1 if no (matching) tag name is found
- abbrev: for defining the length of sha1 returned

Change GitRepository.find_tag() to use the the new method.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>