Use correct latest commit for files

The commit must match *all* parents, otherwise it is not the latest
commit for the file.  This means it must appear in the diff of the
commit against all its parents.

This behaviour has been adapted from the one in the libgit2 log example,
located in the `examples/log.c` file in libgit2.

quit.go

@@ -466,25 +466,31 @@ func (fc *FancyFile) Commit() (*FancyCommit, error) {
				return false
			}
			parentCommit := commit.Parent(0)
			parentTree, err := parentCommit.Tree()
			if err != nil {
				findErr = err
				return false
			}
			diff, err := fc.repo.DiffTreeToTree(parentTree, tree, &opts)
			if err != nil {
				findErr = err
				return false
			matchesAll := true
			for i := uint(0); i < commit.ParentCount(); i++ {
				parentCommit := commit.Parent(i)
				parentTree, err := parentCommit.Tree()
				if err != nil {
					findErr = err
					return false
				}
				diff, err := fc.repo.DiffTreeToTree(parentTree, tree, &opts)
				if err != nil {
					findErr = err
					return false
				}
				n, err := diff.NumDeltas()
				if err != nil {
					findErr = err
					return false
				}
				matchesAll = matchesAll && n > 0
			}
			n, err := diff.NumDeltas()
			if err != nil {
				findErr = err
				return false
			}
			if n <= 0 {
			if !matchesAll {
				return true
			}