git-ff-to

#!/bin/bash

if [ $# -eq 0 ] || [ "$1" == "--help" ] || [ "$1" == "-?" ]; then
    cat <<USAGE_HERE_DOCUMENT
usage: git ff-to [merge-options] <commitish>
Fast forwards the current branch one commit towards the specified commit.

    Given the following history:
        A--B--C--D--E
           \--X--/
    If HEAD is at commit A, then:
    $ git ff-to E   # HEAD == B
    $ git ff-to E   # HEAD == C
    $ git ff-to E   # HEAD == D
    $ git ff-to E   # HEAD == E
USAGE_HERE_DOCUMENT
    exit
fi

MERGE_ARG_LEN=$(($#-1))
MERGE_ARGS=${@:1:$MERGE_ARG_LEN}
TARGET=${@:$#}

CURREV=$(git rev-parse --revs-only --short HEAD) || exit
echo "# Currently at: $(git log -n 1 --oneline --decorate HEAD)"

TARGETREV=$(git rev-parse --revs-only --short "$TARGET") || exit
echo "# FF towards:   $(git log -n 1 --oneline --decorate $TARGET)"

if [ "$CURREV" == "$TARGETREV" ]; then
    echo "Already at target commit!" 1>&2
    exit
fi

NEXTCOMMIT=$(git next-commit "$TARGET")
if [ $? -ne 0 ] || [ -z $NEXTCOMMIT ]; then
    echo "Could not determine next commit!" 1>&2
    exit
fi
echo "# Next commit:  $(git log -n 1 --oneline --decorate $NEXTCOMMIT)"

if [ -z $MERGEARGS ]; then git merge --ff-only $NEXTCOMMIT
else git merge --ff-only $MERGE_ARGS $NEXTCOMMIT
fi