panic in libgit2 ResolveRevision

[sqs]

From the production logs:

2014/09/15 20:41:12 http: panic serving 1.2.3.4:5678: runtime error: invalid memory address or nil pointer dereference
goroutine 19 [running]:
net/http.func·011()
        /usr/local/go/src/pkg/net/http/server.go:1100 +0xb7
runtime.panic(0x7c19c0, 0xc2f813)
        /usr/local/go/src/pkg/runtime/panic.c:248 +0x18d
github.com/sourcegraph/go-vcs/vcs/git_libgit2.(*GitRepositoryLibGit2).ResolveRevision(0xc2081bd0e0, 0xc2098d9a04, 0x28, 0x0, 0x0, 0x0, 0x0)
        /home/sqs/src/github.com/sourcegraph/vcsstore/Godeps/_workspace/src/github.com/sourcegraph/go-vcs/vcs/git_libgit2/git_libgit2.go:61 +0x174
github.com/sourcegraph/vcsstore/server.(*Handler).serveRepoRevision(0xc2080db800, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040, 0x0, 0x0)
        /home/sqs/src/github.com/sourcegraph/vcsstore/server/rev.go:48 +0x198
github.com/sourcegraph/vcsstore/server.*Handler.(github.com/sourcegraph/vcsstore/server.serveRepoRevision)·fm(0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040, 0x0, 0x0)
        /home/sqs/src/github.com/sourcegraph/vcsstore/server/handler.go:55 +0x56
github.com/sourcegraph/vcsstore/server.handler.ServeHTTP(0xc2080db800, 0xc20810afc0, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        /home/sqs/src/github.com/sourcegraph/vcsstore/server/handler.go:74 +0x54
github.com/sourcegraph/vcsstore/server.(*handler).ServeHTTP(0xc20810afd0, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        <autogenerated>:6 +0xae
github.com/sqs/mux.(*Router).ServeHTTP(0xc208030910, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        /home/sqs/src/github.com/sourcegraph/vcsstore/Godeps/_workspace/src/github.com/sqs/mux/mux.go:98 +0x297
github.com/sourcegraph/vcsstore/server.(*Handler).ServeHTTP(0xc2080db800, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        /home/sqs/src/github.com/sourcegraph/vcsstore/server/handler.go:64 +0x49
net/http.(*ServeMux).ServeHTTP(0xc20803e8a0, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        /usr/local/go/src/pkg/net/http/server.go:1511 +0x1a3
net/http.serverHandler.ServeHTTP(0xc208035e60, 0x7f9b3e5b1f28, 0xc2080dd900, 0xc2088ef040)
        /usr/local/go/src/pkg/net/http/server.go:1673 +0x19f
net/http.(*conn).serve(0xc2080c0680)
        /usr/local/go/src/pkg/net/http/server.go:1174 +0xa7e
created by net/http.(*Server).Serve
        /usr/local/go/src/pkg/net/http/server.go:1721 +0x313