Restores the branchnames of all merged branches between two revisions

git-restore-branches.sh

#!/bin/bash
#
# Restores the branchnames of all merged branches
# between two revisions,
# and prints them to stdout.
#
# If the end revision is omitted it defaults to HEAD.
#
# If a branchname already exists,
# it is not moved,
# the branchname is not printed to stdout,
# and an error is printed to stderr.

# Prints usage to stderr and exits with error code 1.
usage()
{
    errcho "git restore-branches <from-rev> [<to-rev>]"
    exit 1
}

# Echos all arguments to stderr as a single string.
errcho()
{
    >&2 echo "$@"
}

# Extracts the branchname from a standard formatted merge commit.
#
# Arguments:
# - outvar: the name of an out-variable to store the branchname in
# - string: the merge commit rev
branchname_from_merge_commit()
{
    local _outvar=$1
    local _merge_commit="$2"
    eval $_outvar="$(git log -1 --format=%s "${_merge_commit}" | \
        awk -F\' '{print $2}')"
}

# Outputs the sha1s of all merge commits
# and the last commit on the merged-in branch
# in the given Git revision range.
#
# Arguments:
# - string: a valid Git revision range
merge_points_between()
{
    local _range="$1"
    git rev-list --merges --parents "${_range}" | awk '{print $1, $3}'
}

# Outputs the sha1 of a valid Git revision.
#
# Arguments:
# - string: a valid Git revision
sha1_of_rev()
{
    local _rev="$1"
    git rev-parse --verify "$_rev" 2>/dev/null
}

[[ $# -eq 0 ]] && usage

from="$1"
to="${2:-HEAD}"

while read merge_commit last_branch_commit; do
    branchname_from_merge_commit branchname "$merge_commit"

    if git branch "$branchname" "$last_branch_commit" 2>/dev/null; then
        echo $branchname
    else
        sha1="$(sha1_of_rev "$branchname")"
        errcho "skipped '${branchname}': already exists at ${sha1}"
    fi
done < <(merge_points_between "${from}..${to}")