Add cms-rebase-topic and automatic backup branches

[kpedro88]

• add a tool cms-rebase-topic that is an extension of cms-checkout-topic that calls rebase after checking out the branch
• extended rebase syntax (--onto) can be activated using the old-base option (in case of backports, diverging branches, etc.); the specified old base is also used for the merge-base calculation (in any of the related tools; sometimes it's nice to checkout a branch in an old release without getting every package in CMSSW)
• cms-rebase-topic and cms-merge-topic create a backup branch (named ..._backup) by default before rebasing or merging, to standardize recovery instructions for users
• actually ran make to update the man pages from source (which hadn't been done in a while)

[cmsbuild]

A new Pull Request was created by @kpedro88 (Kevin Pedro) for branch master.

@cmsbuild, @smuzaffar, @mrodozov 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. You can reject by replying to this message having '-1' in the first line of your reply.

external issue cms-sw/cmsdist#3325

[smuzaffar]

looks good