I threw together a small utility called "git-reparent", available on GitHub at:
https://github.com/MarkLodato/git-reparent
I welcome any comments or suggestions. To make discussion easier,
I've copied the README and code below.
--- 8< ---
NAME
====
git-reparent - Recommit HEAD with a new set of parents.
SYNOPSIS
========
``git reparent [OPTIONS] ((-p <parent>)... | --no-parent)``
DESCRIPTION
===========
Create a new commit object that has the same tree and commit message as HEAD
but with a different set of parents. If ``--no-reset`` is given, the full
object id of this commit is printed and the program exits; otherwise, ``git
reset`` is used to update HEAD and the current branch to this new commit.
This command can be used to manually shape the history of a repository.
Whereas ``git rebase`` moves around *diffs*, ``git reparent`` moves around
*snapshots*. See EXAMPLES for a reason why you might want to use this
command.
OPTIONS
=======
-h, --help show the help
-e, --edit edit the commit message in an editor first
-m, --message <message> use the given message instead of that of HEAD
-p, --parent <commit> new parent to use; may be given multiple times
--no-parent create a parentless commit
-q, --quiet be quiet; only report errors
--no-reset print the new object id instead of updating HEAD
INSTALLATION
============
Make executable and place somewhere in your $PATH.
EXAMPLES
========
Reparenting the tip of a branch
-------------------------------
Suppose we create some commit *B* and then accidentally pass the ``--amend``
flag when creating new commit *C*, resulting in the following history::
B
/
...---A---C (HEAD)
What we really wanted was one linear history, ``...---A--B--C``. If we
were to use ``git rebase`` or ``git cherry-pick`` to reconstruct the history,
this would try to apply the *diff* of *A..C* onto *B*, which might fail.
Instead, what we really want to do is use the exact message and tree from *C*
but with parent *B* instead of *A*. To do this, we run ::
$ git reparent -p B
where the name *B* can be found by running ``git reflog`` and looking for the
first "commit (amend)". The resulting history is now just what we wanted::
C
/
...---A---B---C' (HEAD)
Reparenting an inner commit
---------------------------
We can also update the parents of a commit other than the most recent.
Suppose that we want to perform a rebase-like operation, moving *master* onto
*origin/master*, but we want to completely ignore any changes made in the
remote branch. That is, our history currently looks like this::
B---C (master, HEAD)
/
...---A---D---E (origin/master)
and we want to make it look like this::
B---C (origin/master)
/ /
...---A---D---E---B'---C' (master, HEAD)
We can accomplish this by using ``git rebase --interactive`` along with ``git
reparent``::
$ git rebase -i A
# select the "edit" command for commit B
# git rebase will dump us out at commit B
$ git reparent -p origin/master
$ git rebase --continue
Now the history will look as desired, and the trees, commit messages, and
authors of *B'* and *C'* will be identical to those of *B* and *C*,
respectively.
SEE ALSO
========
git-filter-branch(1) combined with either git grafts or git-replace(1) can be
used to achieve the same effect
git-rebase(1) can be used to re-apply the *diffs* of the current branch to
another
AUTHOR
======
Mark Lodato <lodatom@xxxxxxxxx>
--- 8< ---
#!/bin/sh
# Copyright (c) Mark Lodato, 2013
OPTIONS_SPEC="\
git reparent [OPTIONS] ((-p <parent>)... | --no-parent)
Recommit HEAD with a new set of parents.
--
h,help show the help
e,edit edit the commit message in an editor first
m,message= use the given message instead of that of HEAD
p,parent=! new parent to use; may be given multiple times
no-parent! create a parentless commit
q,quiet be quiet; only report errors
reset* default behavior
no-reset! print the new object id instead of updating HEAD
"
SUBDIRECTORY_OK=Yes
. "$(git --exec-path)/git-sh-setup" || exit $?
require_clean_work_tree reparent "Please commit or stash them first."
# Location of the temporary message.
msg_file="$GIT_DIR/reparent-msg"
die_with_usage() {
echo "error: $1" >&2
usage
}
edit=
message=
no_parent=
no_reset=
parent_flags=
quiet=
while [ $# -gt 0 ]; do
case "$1" in
-p)
[ $# -eq 0 ] && die_with_usage "-p requires an argument"
shift
parent_flags="$parent_flags$(git rev-parse --sq-quote -p "$1")"
;;
--no-parent) no_parent=1 ;;
-e) edit=1 ;;
--no-edit) edit= ;;
-m) message="$2"; shift ;;
--no-message)message= ;;
-q) quiet=-q ;;
--no-quiet) quiet= ;;
--reset) no_reset= ;;
--no-reset) no_reset=1 ;;
--) shift; break ;;
*) die "internal error: unknown flag $1" ;;
esac
shift
done
[ $# -gt 0 ] && \
die_with_usage "no positional arguments expected"
[ -z "$no_parent" -a -z "$parent_flags" ] && \
die_with_usage "either -p or --no-parent is required"
[ -n "$no_parent" -a -n "$parent_flags" ] && \
die_with_usage "-p and --no-parent are mutually exclusive"
# Create the commit.
if [ -n "$message" ]; then
echo "$message" > "$msg_file"
else
git cat-file commit HEAD | sed "1,/^$/d" > "$msg_file"
fi
if [ -n "$edit" ]; then
# TODO: use the normal `git commit` comment stripping stuff
git_editor "$msg_file" || die "no editor configured"
[ -s "$msg_file" ] || die "aborting due to empty commit message"
fi
eval "$(get_author_ident_from_commit HEAD)"
old_head="$(git rev-parse --short HEAD)" || exit $?
new_head="$(eval "git commit-tree HEAD: $parent_flags" '< $msg_file')" || \
exit $?
rm "$msg_file"
# Print out the commit if --no-reset; otherwise update HEAD.
if [ -n "$no_reset" ]; then
echo "$new_head"
else
set_reflog_action reparent
git reset $quiet "$new_head" || exit $?
new_abbrev="$(git rev-parse --short HEAD)" || exit $?
[ -z "$quiet" ] && echo "Moved HEAD to $new_abbrev (was $old_head)"
fi
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