hgeditor

rebase: do not invent successor to skipped changeset

When rebase results in an empty a changeset it is "skipped" and no related
changeset is created at all. When we added obsolescence support to rebase (in
fc2a6114f0a0) it seemed a good idea to use its parent successor as the
successors for such dropped changesets. (see old version of the altered test).
This option was chosen because it seems a good way to hint about were the
dropped changeset "intended" to be. Such hint would have been used by automatic
evolution mechanism to rebase potential unstable children.

However, field testing of this version are not conclusive. It very often leads
to the creation of (totally unfounded) evolution divergence. This changeset
changes this behavior and mark skipped changesets as pruned (obsolete without
successors). This prevents the issue and seems semantically better probably a
win for obsolescence reading tool.

See example bellow for details:

User Babar has five changesets of interest:
- O, its current base of development.
- U, the new upstream
- A and C, some development changesets
- B another development changeset independent from A

    O - A - B - C
      \
        U

Babar decides that B is more critical than the A and C and rebase it first

  $ hg rebase --rev B --dest U

B is now obsolete (in lower case bellow). Rebase result, B', is its
successors.(note, C is unstable)

    O - A - b - C
      \
        U - B'

Babar is now done with B', and want to rebase the rest of its history:

  $ hg rebase --source A --dest B'


hg rebase process A, B and C. B is skipped as all its changes are already contained
in B'.

    O - U - B' - A' - C'

Babar have the expected result graph wise, obsolescence marker are as follow:

    B -> B' (from first rebase)
    A -> A' (from second rebase)
    C -> C' (from second rebase)
    B -> ?? (from second rebase)

Before this changeset, the last marker is `B -> A'`. This cause two issues:

- This is semantically wrong. B have nothing to do with A'
- B has now two successors sets: (B',) and (A',). We detect a divergent
  rewriting. The B' and A' are reported as "divergent" to Babar, confusion
  ensues. In addition such divergent situation (divergent changeset are children
  to each other) is tricky to solve.

With this changeset the last marker is `B -> ø`:

- This is semantically better.
- B has a single successors set (B',)

This scenario is added to the tests suite.

#!/bin/sh
#
# This is an example of using HGEDITOR to create of diff to review the
# changes while commiting.

# If you want to pass your favourite editor some other parameters
# only for Mercurial, modify this:
case "${EDITOR}" in
    "")
        EDITOR="vi"
        ;;
    emacs)
        EDITOR="$EDITOR -nw"
        ;;
    gvim|vim)
        EDITOR="$EDITOR -f -o"
        ;;
esac


HGTMP=""
cleanup_exit() {
    rm -rf "$HGTMP"
}

# Remove temporary files even if we get interrupted
trap "cleanup_exit" 0 # normal exit
trap "exit 255" HUP INT QUIT ABRT TERM

HGTMP=$(mktemp -d ${TMPDIR-/tmp}/hgeditor.XXXXXX)
[ x$HGTMP != x -a -d $HGTMP ] || {
  echo "Could not create temporary directory! Exiting." 1>&2
  exit 1
}

(
    grep '^HG: changed' "$1" | cut -b 13- | while read changed; do
        "$HG" diff "$changed" >> "$HGTMP/diff"
    done
)

cat "$1" > "$HGTMP/msg"

MD5=$(which md5sum 2>/dev/null) || \
    MD5=$(which md5 2>/dev/null)
[ -x "${MD5}" ] && CHECKSUM=`${MD5} "$HGTMP/msg"`
if [ -s "$HGTMP/diff" ]; then
    $EDITOR "$HGTMP/msg" "$HGTMP/diff" || exit $?
else
    $EDITOR "$HGTMP/msg" || exit $?
fi
[ -x "${MD5}" ] && (echo "$CHECKSUM" | ${MD5} -c >/dev/null 2>&1 && exit 13)

mv "$HGTMP/msg" "$1"

exit $?