git-svn: added an --include-path flag

The SVN::Fetcher module is now able to filter for inclusion as well as exclusion (as used by --ignore-path). Also added tests, documentation changes and git completion script.

Signed-off-by: Paul Walmsley <paul@pjwhams.co.uk>
Merge branch 'master-git-svn-merge2' of https://github.com/pjwhams/git into master-git-svn-merge2

Conflicts:
	perl/Git/SVN/Fetcher.pm
	t/t9147-git-svn-include-paths.sh