Adds support for .gitignore files to git_status_foreach() and
git_status_file(). This includes refactoring the gitattributes
code to share logic where possible. The GIT_STATUS_IGNORED flag
will now be passed in for files that are ignored (provided they
are not already in the index or the head of repo).
Add git_status_hashfile() to get blob's object id for a file without adding
it to the object database or needing a repository at all.
This functionality is similar to `git hash-object` without '-w'.
