'tagger' value should be assigned to both Author and Committer probably

[SagePtr]

In cat-file output, regular commits have following keys:

author (one who wrote code and sent diff content)
committer (one who commited that code to repo or merged PR)

Tag commits have one key: tagger (which corresponds to user who made new tag)

Now, in repo_commit.go, function parseCommitData, tagger is assigned to commit.Author, and commit.Committer stays nil.

I think, for better compatibility, tagger should be assigned to both commit.Author and commit.Committer, because in this case tagger is both author of tag and committer of this tag.

This would also solve go-gitea/gitea#4961 issue when signatures are verified against Committer field, but Committer field is just nil for tag commit.