Auto merge of #1122 - alexcrichton:issue-1119, r=brson

It looks like tags don't always point to tag objects, but rather sometimes to
commits themselves. This tweaks the peeling logic for tags to find an *object*
first, and then peel to a commit, rather than finding a tag first and then
peeling.

Closes #1119

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