Use the ref for the commit link for commits against tags.
Use the ref for the commit link for commits against tags.

Builders that build against the tag commits benefit from having the tag
ref displayed instead of a commit hash since the tag identifies what's
being built in a more human-recognizable form. Currently, the tag ref
will be used only if the output commit does not have an ID set. The
common depot tools code always sets the ID when setting the output
commit, so for chromium builders building against tags, additional
configuration is used to prevent them from setting the output commit.
Displaying the tag in that case will allow for chromium builders to
unconditionally set the output commit, which will simplify configuration
code and queries of builds.

Bug: 1174266
Change-Id: I1afdd25e96521ef3cef92a23e9e13e04bbeae16c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3591176
Reviewed-by: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Commit-Queue: Garrett Beaty <a0767768af30bcbe01b6de5024355c539362a830@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go