scripts/validate: Fix git rev-parse when doing local builds

When doing local builds (i.e. without $DRONE_REPO and $DRONE_PULL_REQUEST,
or $DRONE_COMMIT_REF set), the build fails with "fatal: Needed a single
revision" in `scripts/validate`:

```
> make REPO=tserong
[...]
Running validation
Running: go vet
refs/heads/wip-add-virtualSize
fatal: Needed a single revision
FATA[0032] exit status 128
make: *** [Makefile:11: ci] Error 1
```

If I run the commands in `scripts/validate` by hand, I see this:

```
> git symbolic-ref -q HEAD && REV="origin/HEAD" || REV="HEAD^"
refs/heads/wip-add-virtualSize
> echo $REV
origin/HEAD
> headSHA=$(git rev-parse --short=12 ${REV})
fatal: Needed a single revision
```

I don't know if this is just something weird about my environment, but
if I change "origin/HEAD" to "HEAD", it works fine:

```
> headSHA=$(git rev-parse --short=12 HEAD)
> echo $headSHA
eb27fbf6e678
```

Signed-off-by: Tim Serong <[email protected]>

scripts/validate

@@ -19,7 +19,7 @@ elif [ ! -z "${DRONE_COMMIT_REF}" ]; then
 	echo "Running: golangci-lint run --new-from-rev=${DRONE_COMMIT_REF}"
 	golangci-lint run --new-from-rev=${DRONE_COMMIT_REF}
 else
-	git symbolic-ref -q HEAD && REV="origin/HEAD" || REV="HEAD^"
+	git symbolic-ref -q HEAD && REV="HEAD" || REV="HEAD^"
 	headSHA=$(git rev-parse --short=12 ${REV})
 	echo "Running: golangci-lint run --new-from-rev=${headSHA}"
 	golangci-lint run --new-from-rev=${headSHA}