reuse_merge: script to reuse a previous merge during a rebase

reuse_merge

#!/bin/bash
set -e

# WARNING: This script will delete any untracked files present in the working tree

# Usage: reuse_merge <commit>
# where <commit> is a merge commit whose parents have identical contents to HEAD and MERGE_HEAD (which the script will check).
# This will reuse the state of the supplied merge to make a new merge commit on top of HEAD and MERGE_HEAD.
# This is useful when using `git rebase --rebase-merges`, when reworking commits between merges

# Licence: CC0

# Extract parent commits of the merge commit to reuse
joined_parents=`git show $1 | head -2 | grep -P "^Merge: " | sed -e 's/^Merge: //'`
IFS=' '
read -ra parents<<<"${joined_parents}"
if [ ${#parents[@]} -ne 2 ]; then
   echo "Error: the commit $1 does not appear to be a merge commit with two parents"
   exit 1
fi

echo "Merge commit $1 has parents:"
echo ${parents[0]}
echo ${parents[1]}

# Check that the parents have the same contents as the current commits to merge
function check_commits_have_equal_contents () {
     if [ $1 != $2 ]; then
        diff_size=`git diff $1 $2 | wc -l`
        if [ ${diff_size} -ne 0 ]; then
           echo "Error: commits $1 and $2 differ, cannot re-apply the same merge commit!"
           exit 1
        fi
     fi
}

check_commits_have_equal_contents ${parents[0]} HEAD
check_commits_have_equal_contents ${parents[1]} MERGE_HEAD

echo "Parents are identical, merging"

rm -r *
git checkout $1 .
git add .
git commit -C $1