Changed the way detect_presence works with git.

A test was failing due to the previous implementation.
See test/test_git_subm.py:147
Changed the way detect_presence works with git.

A test was failing due to the previous implementation.
See test/test_git_subm.py:147
Added a comment pointing to the github issue where the alternatives were discussed.
Merge pull request #10 from vcstools/git_detect_presence_fix

git: changed the way detect_presence works

Previously it checked for the existence of a .git directory, but in newer versions of git submodules have a .git file, so now it just checks that either a file or a directory called .git exists.
Update doc/changelog.rst

Change log entry for pull request #10
git: Fixed a problem where checkout always returned True even when there was an invalid version.

If you call 'git_client.checkout(valid_url, nonexistent_branch)' it will print an error that the git checkout command failed because the branch isn't there, but checkout will still return true.
Merge pull request #13 from vcstools/checkout_update_error_return_fix

git: Fixed a problem where checkout always returned True
svn: bug fix in the export_repositories command
reving to 0.1.22
hack so that git._do_fast_forward returns false when in diverged state (and fast-forward is not performed
refactor no duplicate calls to get_branch_parent
bugfix: fast_forward succeeds when on same commit or local moved on (rev-list command not properly used)
added unit-test for diverge
better open/close
no filter also does not filter std, since svn prints there regarding certificates
no filter for svn operations
fixed Makefile
0.1.23
Merge pull request #16 from tkruse/certificates

show prompt for Certificates for svn
added hack to enable forcing fetch with --tags on git update
0.1.24