git-mergediff.sh

#!/bin/bash

#
# preferences


# set DEBUG to "-v" or "" -- it will
#   1. activate shell tracing
#   2. be passed to curl as an argument
if [ -z "$DEBUG" ];
then
    DEBUG=""
fi

#
# preconditions

set -e
if [ ! -z $DEBUG ];
then
    set -x
fi

require () {
    if ! command -v $1 > /dev/null 2>&1 ;
    then
        echo "Requires $1$2. Install $1 and rerun."
        exit
    fi
}

require git


#
# help command

if [ "$1" == "help" -o "$1" == "--help" ] ;
then
    cat <<EOF

mergediff [srcref] targetref

Sets the state of the working tree to <targetref> and HEAD to <srcref>.
Uses current branch as the source reference when the argument is omitted.

Note that any changes in the working in the working tree when this command is run may result in undefined behavior.

EXAMPLE:

Show remote branch 'my-pr' as a diff against develop:
git mergediff develop origin/my-pr

Show remote branch 'my-pr' as a diff against the current branch:
git mergediff origin/my-pr


EOF

    exit
fi

git fetch --all

if [ "$#" == "2" ] ;
then
    SRCBRANCH=$1
    shift
fi

TARGETBRANCH=$1

if [ ! -z "$SRCBRANCH" ];
then
    echo
    echo "Will run: git checkout $SRCBRANCH && git reset --hard"
    read -p"Press enter to continue"
    git checkout $SRCBRANCH && git reset --hard
fi

SRCREF=`git symbolic-ref HEAD`

git checkout $TARGETBRANCH
git symbolic-ref HEAD $SRCREF
git reset

git status -s
exit

#
# fallback to help command
$0 --help