[SPARK-28496][INFRA] Use branch name instead of tag during dry-run

## What changes were proposed in this pull request?

There are two cases when we use `dry run`.

First, when the tag already exists, we can ask `confirmation` on the existing tag name.
```
$ dev/create-release/do-release-docker.sh -d /tmp/spark-2.4.4 -n -s docs
Output directory already exists. Overwrite and continue? [y/n] y
Branch [branch-2.4]:
Current branch version is 2.4.4-SNAPSHOT.
Release [2.4.4]: 2.4.3
RC # [1]:
v2.4.3-rc1 already exists. Continue anyway [y/n]? y
This is a dry run. Please confirm the ref that will be built for testing.
Ref [v2.4.3-rc1]:
```

Second, when the tag doesn't exist, we had better ask `confirmation` on the branch name. If we do not change the default value, it will fail eventually.
```
$ dev/create-release/do-release-docker.sh -d /tmp/spark-2.4.4 -n -s docs
Branch [branch-2.4]:
Current branch version is 2.4.4-SNAPSHOT.
Release [2.4.4]:
RC # [1]:
This is a dry run. Please confirm the ref that will be built for testing.
Ref [v2.4.4-rc1]:
```

This PR improves the second case by providing the branch name instead. This helps the release testing before tagging.

## How was this patch tested?

Manually do the following and check the default value of `Ref` field.
```
$ dev/create-release/do-release-docker.sh -d /tmp/spark-2.4.4 -n -s docs
Branch [branch-2.4]:
Current branch version is 2.4.4-SNAPSHOT.
Release [2.4.4]:
RC # [1]:
This is a dry run. Please confirm the ref that will be built for testing.
Ref [branch-2.4]:
...
```

Closes apache#25240 from dongjoon-hyun/SPARK-28496.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>
(cherry picked from commit cfca26e)
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

dev/create-release/release-util.sh

@@ -150,6 +150,9 @@ function get_release_info {
   GIT_REF="$RELEASE_TAG"
   if is_dry_run; then
     echo "This is a dry run. Please confirm the ref that will be built for testing."
+    if [[ $SKIP_TAG = 0 ]]; then
+      GIT_REF="$GIT_BRANCH"
+    fi
     GIT_REF=$(read_config "Ref" "$GIT_REF")
   fi
   export GIT_REF