extract common process execution to a method

[jtnord]

almost the same code was called 3 times in a row when cloning, extracted to a common method to call.

validated the failure case manually with an old plugin using the git:// protocol

failed with

2023-04-28 17:21:26.405+0000 [id=1]     INFO    o.j.t.test.PluginCompatTester#cloneImpl: Checking out from Git repository git://github.com/jenkinsci-cert/secure-requester-whitelist-plugin.git at secure-requester-whitelist-1.7
org.jenkins.tools.test.exception.PluginSourcesUnavailableException: git fetch git://github.com/jenkinsci/secure-requester-whitelist-plugin.git secure-requester-whitelist-1.7 failed with exit status 128: fatal: unable to connect to github.com:
github.com[0: 140.82.113.4]: errno=Unknown error
        at org.jenkins.tools.test.PluginCompatTester.runCommand(PluginCompatTester.java:574)

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Thanks for the PR!