git-redo-merge

#!/usr/bin/env bash

prog="$(basename $0)"

if [ $# -gt 0 ]; then
    cat <<EOF
  Usage:
   \$ $prog

  Redo a merge after a failed attempt to push it due to
  the tracked upstream branch being ahead.

  Example:
    (master)\$ git merge topic
    (master)\$ git push upstream master
    FAIL.
    ! [rejected]         master -> master (non-fast-forward)

    (master)\$ $prog
    (master)\$ git push upstream master
    Ok

EOF
    exit 1
fi


curr_branch="$(git rev-parse --symbolic-full-name --abbrev-ref HEAD)"

if [ "$curr_branch" == "" ]; then
    echo "$prog ERROR: No branch checked out. You are on a detached HEAD."
    exit 1
fi

if ! git diff --quiet --exit-code; then
    echo "$prog ERROR: Repo is dirty. You have unstaged changes."
    exit 1
fi

if ! git diff --staged --quiet --exit-code HEAD; then
    echo "$prog ERROR: Repo is dirty. You have staged changes."
    exit 1
fi

remote_branch="$(git rev-parse --symbolic-full-name @{u} 2> /dev/null)"
if [ $? -ne 0 -o "$remote_branch" == "" ]; then
    echo "$prog ERROR: Branch '$curr_branch' is not tracking any remote branch."
    exit 1
fi
echo "$prog: Using remote bransh '$remote_branch'"


old_ours="$(git rev-parse --verify --quiet HEAD^1)"
if [ $? -ne 0 -o "$old_ours" == "" ]; then
    echo "$prog ERROR: HEAD does not even have a first parent???"
    exit 1
fi

old_theirs="$(git rev-parse --verify --quiet HEAD^2)"
if [ $? -ne 0 -o "$old_theirs" == "" ]; then
    echo "$prog ERROR: HEAD is not a merge commit."
    exit 1
fi

if git rev-parse --verify --quiet HEAD^3; then
    echo "$prog ERROR: HEAD is an octopus merge."
    exit 1
fi

if ! git merge-base --is-ancestor $old_ours $remote_branch; then
    echo "$prog ERROR: First parent HEAD^1 is not an ancestor of $remote_branch."
    exit 1
fi

if git merge-base --is-ancestor $remote_branch $curr_branch; then
    echo "$prog ERROR: $remote_branch is reachable from $curr_branch. They have not diverged."
    exit 1
fi

if git merge-base --is-ancestor $curr_branch $remote_branch; then
    echo "$prog ERROR: $curr_branch is reachable from $remote_branch. They have not diverged."
    exit 1
fi

old_head="$(git rev-parse --verify --quiet HEAD)"
if [ $? -ne 0 -o "$old_head" == "" ]; then
    echo "$prog ERROR: Cannot rev-parse HEAD???"
    exit 1
fi

if ! git merge --no-edit $remote_branch; then
    echo
    echo "$prog:"
    echo -e "\tFailed to merge $remote_branch due to conflicts."
    echo -e "\tEither solve conflicts, commit merge and do the fixup-merge yourself"
    echo -e "\tor do \"git merge --abort\" to get back where you started."
    exit 1
fi


# Fixup merge
git reset --soft $remote_branch
git update-ref MERGE_HEAD $old_theirs
git commit --reuse-message=$old_head