Check mergedtruth is in the sequence before modifying it

[mark-grimes]

If DIGI is run instead of DIGI:pdigi_valid then mergedtruth is not in the sequence. This just sets the customisation to check first.

[cmsbuild]

A new Pull Request was created by @mark-grimes (Mark Grimes) for CMSSW_6_2_X_SLHC.

Check mergedtruth is in the sequence before modifying it

It involves the following packages:

SLHCUpgradeSimulations/Configuration

@cmsbuild, @civanch, @Degano, @mdhildreth, @nclopezo can you please review it and eventually sign? Thanks.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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@fratnikov, @mark-grimes you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[mark-grimes]

merge

Tested all configurations just in case. Everything fine and extended pixel now runs without pdigi_valid.