A helper for interactive rebases with conflicts

Team,

I frequently find my interactive rebases being interrupted with conflicts
due to already-upstreamed patches. Typically, these patches should be left
out from the todo list (because it uses the `right_only` flag of the
revision walker), but that logic can fail when passing an `--onto`
parameter: the upstreamed patch might not be part of the symmetric range
used to generate the todo list.

For these cases, I wrote this little script, which looks at the oneline of
the commit which could not be cherry-picked, tries to find it in the
commits reachable from HEAD (but not from the cherry-picked commit), and
once found, runs a range-diff:

-- snip --
#!/bin/sh

string2regex () {
	echo "$1" | sed 's/[]\\\/[*?]/\\&/g'
}

compare_to_upstream_commit () { # [<tip-commit>] [<upstream-branch>] [<commit-count>]
	tip=HEAD
	upstream=upstream/master
	count=1

	test $# -gt 0 ||
	test ! -f "$(git rev-parse --git-path rebase-merge/stopped-sha)" ||
	set -- stopped-sha

	case "$1" in
	stopped|stopped-sha)
		tip="$(cat "$(git rev-parse --git-path rebase-merge/stopped-sha)")" &&
		git rev-parse -q --verify "$tip" ||
		die "Could not get stopped-sha; Not in a rebase?\n"
		upstream=HEAD
		shift
		;;
	*[^0-9a-f]*) ;; # not a tip commit
	?*) tip="$(git rev-parse --verify "$1"^0)" || die "Could not parse $1"; shift;;
	esac

	case "$1" in
	*[^0-9]*) upstream="$1"; shift;;
	esac

	case "$1" in
	''|*[^0-9]*) ;; # not a count
	*) count=$1; shift;;
	esac

	test 0 = $# ||
	die "Unhandled argument(s): $*"

	oneline="$(git show -s --format=%s $tip)" &&
	regex="$(string2regex "$oneline")" &&
	upstream_commit="$(git rev-list --no-merges -1 --grep="$regex" $tip.."$upstream")" &&
	{ test -n "$upstream_commit" ||
		upstream_commit="$(git rev-list --no-merges -1 -i --grep="$regex" $tip.."$upstream")"; } &&
	{ test -n "$upstream_commit" ||
		upstream_commit="$(git range-diff -s $tip^..$tip $tip.."$upstream" |
			sed -n 's/^[^:]*:[^:]*=[^:]*: *\([^ ]*\).*/\1/p')"; } &&
	{ test -n "$upstream_commit" ||
		upstream_commit="$(git range-diff --creation-factor=95 -s $tip^..$tip $tip.."$upstream" |
			sed -n 's/^[^:]*:[^:]*=[^:]*: *\([^ ]*\).*/\1/p')"; } &&
	test -n "$upstream_commit" ||
	die "Could not find upstream commit for '$oneline'"

	git range-diff --creation-factor=95 "$tip~$count..$tip" "$upstream_commit~$count..$upstream_commit"
}

compare_to_upstream_commit "$@"
-- snap --

As you can see, this script does more than just handle interrupted
rebases: it also allows you to specify a tip commit, an upstream branch
and a commit count to conveniently find the tip commit in the upstream
branch and then do a range-diff of <commit-count> patches.

Obviously, the described logic fails badly when the oneline has been
changed, so the script falls back to running a full range-diff and seeing
whether it can identify the upstream commit that way. If that fails, too,
it runs the range-diff again with a looser net.

Maybe this script will prove useful to somebody else than me, too?

Ciao,
Johannes