commit-msg: Invoke gerrit/commit-msg if hooks.GerritId=true
commit-msg: Invoke gerrit/commit-msg if hooks.GerritId=true

Add a Gerrit Change-Id header if hooks.GerritId is true.  Print help if
the option is not set and a remote URL looks like Gerrit.

Sprunth/RGG,Sprunth/RGG,Sprunth/RGG,Sprunth/RGG