fix(shell) Fix target with an SCM to correctly add the changed files by the shell command

[dduportal]

Fix #342

This PR will unblocks jenkinsci/bom#698.

A manual test had been run , resulting in dduportal/bom#2.

Please note that this PR should only result on a patch update (no breaking change, no new feature: only a bugfix)

Test

To test this pull request, you can run the following commands:

# Unit Tests
  cp pkg/plugins/shell
  go test

# End to End
go build -o dist/updatecli
updatecli diff (...) # in your failing context

Additional Information

• This solution was found with a common brainstomring with @timja and @olblak : thanks folks!
• Please note that it changes the interface of the scm.Scm type: it is required to allow mock injection during the unit tests (otherwise we could have only used the git/generic method)

[olblak]

I am a bit concerned by adding every file without checking their status but we can still improve that later

[timja]

nice work, fyi @jetersen