Auto merge of #3583 - alexcrichton:fix-tags, r=brson

Handle `rev` being a `tag`

Previously if a rev was specified as a tag then we'd trip an assertion because
resetting to that tag would reset to the tag that the commit pointed to, which
would then cause the head id of the repo to be different than what we thought it
was.

Instead, we handle the case where a `rev` specification is a tag explicitly by
using the tag's target id as the revision that we're going to check out, not the
id of the tag itself.

Closes #3580

diff --cc src/cargo/sources/git/utils.rs
Simple merge