perf: run "rebase -i" under perf

This developer spent a lot of time trying to speed up the interactive
rebase, in particular on Windows. And will continue to do so.

To make it easier to demonstrate the performance improvement, let's have
a reproducible performance test.

The topic branch we use to test performance was found using these shell
commands (essentially searching for a long-enough topic branch in Git's
own history that touched the same file multiple times):

git rev-list --parents origin/master |
grep ' .* ' |
while read commit rest
do
patch_count=$(git rev-list --count $commit^..$commit^2)
test $patch_count -gt 20 || continue

merges="$(git rev-list --parents $commit^..$commit^2 |
grep ' .* ')"
test -z "$merges" || continue

patches_per_file="$(git log --pretty=%H --name-only \
$commit^..$commit^2 |
grep -v '^$' |
sort |
uniq -c -d |
sort -n -r)"
test -n "$patches_per_file" &&
test 20 -lt $(echo "$patches_per_file" |
sed -n '1s/^ *\([0-9]*\).*/\1/p') || continue

printf 'commit %s\n%s\n' "$commit" "$patches_per_file"
done

Note that we can get away with *not* having to reset to the original
branch tip before rebasing: we switch the first two "pick" lines every
time, so we end up with the same patch order after two rebases, and the
complexity of both rebases is equivalent.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>