Replace gogits git-module dependency

[tboerger]

We want to replace the current dependency to gogits/git-module with go-gitea/git, below you can see a list of greped values.

# grep -R git-module *
cmd/serve.go:	git "github.com/gogits/git-module"
models/action.go:	"github.com/gogits/git-module"
models/git_diff.go:	"github.com/gogits/git-module"
models/git_diff.go:// TODO: move this function to gogits/git-module
models/git_diff.go:// TODO: move this function to gogits/git-module
models/pull.go:	"github.com/gogits/git-module"
models/release.go:	"github.com/gogits/git-module"
models/repo.go:	git "github.com/gogits/git-module"
models/repo_branch.go:	"github.com/gogits/git-module"
models/repo_editor.go:	git "github.com/gogits/git-module"
models/update.go:	git "github.com/gogits/git-module"
models/user.go:	"github.com/gogits/git-module"
models/webhook_slack.go:	"github.com/gogits/git-module"
models/wiki.go:	"github.com/gogits/git-module"
modules/context/repo.go:	"github.com/gogits/git-module"
routers/api/v1/convert/convert.go:	"github.com/gogits/git-module"
routers/api/v1/repo/file.go:	"github.com/gogits/git-module"
routers/install.go:	"github.com/gogits/git-module"
routers/repo/commit.go:	"github.com/gogits/git-module"
routers/repo/download.go:	"github.com/gogits/git-module"
routers/repo/editor.go:	"github.com/gogits/git-module"
routers/repo/http.go:	git "github.com/gogits/git-module"
routers/repo/middlewares.go:	"github.com/gogits/git-module"
routers/repo/pull.go:	"github.com/gogits/git-module"
routers/repo/repo.go:	"github.com/gogits/git-module"
routers/repo/setting.go:	"github.com/gogits/git-module"
routers/repo/view.go:	"github.com/gogits/git-module"
routers/repo/webhook.go:	git "github.com/gogits/git-module"
routers/repo/wiki.go:	"github.com/gogits/git-module"
vendor/vendor.json:			"path": "github.com/gogits/git-module",