How can I prevent non-fastforward pushes to selected branch(es) in git?

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Here’s an update hook (copy to hooks/update) that I wrote for my own use. This script by default denies all non-fast-forward updates but allows them for explicitly configured branches. It should be easy enough to invert it so that non-fast-forward updates are allowed for all but the master branch.

#!/bin/sh
#
# A hook script to block non-fast-forward updates for branches that haven't
# been explicitly configured to allow it. Based on update.sample.
# Called by "git receive-pack" with arguments: refname sha1-old sha1-new
#
# Config
# ------
# hooks.branch.<name>.allownonfastforward
#   This boolean sets whether non-fast-forward updates will be allowed for
#   branch <name>. By default they won't be.

# --- Command line
refname="$1"
oldrev="$2"
newrev="$3"

# --- Safety check
if [ -z "$GIT_DIR" ]; then
        echo "Don't run this script from the command line." >&2
        echo " (if you want, you could supply GIT_DIR then run" >&2
        echo "  $0 <ref> <oldrev> <newrev>)" >&2
        exit 1
fi

if [ -z "$refname" -o -z "$oldrev" -o -z "$newrev" ]; then
        echo "Usage: $0 <ref> <oldrev> <newrev>" >&2
        exit 1
fi

# --- Check types
# if $newrev is 0000...0000, it's a commit to delete a ref.
zero="0000000000000000000000000000000000000000"
if [ "$newrev" = "$zero" ]; then
        newrev_type=delete
else
        newrev_type=$(git cat-file -t $newrev)
fi

case "$refname","$newrev_type" in
        refs/tags/*,commit)
                # un-annotated tag
                ;;
        refs/tags/*,delete)
                # delete tag
                ;;
        refs/tags/*,tag)
                # annotated tag
                ;;
        refs/heads/*,commit)
                # branch
                # git rev-list doesn't print anything on fast-forward updates
                if test $(git rev-list "$newrev".."$oldrev"); then
                        branch=${refname##refs/heads/}
                        nonfastforwardallowed=$(git config --bool hooks.branch."$branch".allownonfastforward)

                        if [ "$nonfastforwardallowed" != "true" ]; then
                                echo "hooks/update: Non-fast-forward updates are not allowed for branch $branch"
                                exit 1
                        fi
                fi
                ;;
        refs/heads/*,delete)
                # delete branch
                ;;
        refs/remotes/*,commit)
                # tracking branch
                ;;
        refs/remotes/*,delete)
                # delete tracking branch
                ;;
        *)
                # Anything else (is there anything else?)
                echo "hooks/update: Unknown type of update to ref $refname of type $newrev_type" >&2
                exit 1
                ;;
esac

# --- Finished
exit 0

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