cmd/gitlog: add flag for tags
cmd/gitlog: add flag for tags

Turns out I need this in other places, so flag it is.

Change-Id: Ic67bc3d4f451dc5760d42f64787a5dd6b988db23
Reviewed-on: https://go-review.googlesource.com/91516
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/build,golang/build,golang/build,golang/build