There is no possibility to return Jira issue key from a search in a separate field (neither advanced search nor EyeQL search).
However, if what you need exactly is a list of commits related with a Jira issue, you can try to use undocumented REST API (it's an API used by Jira 'development panel' to obtain list of commits and branches from Fisheye):
http://localhost:8060/fecru/rest/remote-link-aggregation/latest/aggregation?globalId=<issue key>
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