[perl #116965] git commit hook to prevent changes on left of merge

# New Ticket Created by  Ricardo SIGNES 
# Please include the string:  [perl #116965]
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# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=116965 >



In the perl.git repo, we usually want merge commits to introduce changes only
on the right.  That is, this is okay:

  *    merge foo branch
  | \
  |  * work on perl foo
  |  * more work on perl foo
  | /
  *    previous unrelated work

...but this is not:

  *    merge bar branch
  | \
  |  * work on perl bar
  |  * more work on perl bar
  *  | work on blead
  | /
  *    previous unrelated work

The second example should be rebased on blead, and then either merged or not
merged.

Changes on the left of the merge should only occur quite rarely, for example
when merging a release branch back to blead, when blead has advanced during the
release process.

We should prevent merges like this unless --force is given.  The warning should
explain what is wrong, and how the user should proceed.

-- 
rjbs

• [perl #116965] git commit hook to prevent changes on left of merge by Ricardo SIGNES