Hook fails on valid squash messages

[gabrielfeo]

Messages auto-generated for squash aren't currently supported:

# This is a combination of 2 commits.
# This is the 1st commit message:

a

# This is the commit message #2:

b

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Fri Apr 1 11:21:10 2022 +0100
#
# interactive rebase in progress; onto 11e86f9
# Last commands done (2 commands done):
#    pick 019360b a
#    squash c442edd b
# No commands remaining.
# You are currently rebasing branch 'fix/error-on-squash-messages' on '11e86f9'.
#
# Changes to be committed:
#	new file:   a
#	new file:   b
#

Currently rebase squash steps fail on the hook with "There must be a blank line between subject and body". Repro on a7dd0dc.