rebase --skip: fix commit message clean up when skipping squash

rebase --skip: fix commit message clean up when skipping squash

During a series of "fixup" and/or "squash" commands, the interactive
rebase accumulates a commit message from all the commits that are being
squashed together. If one of the commits has conflicts when it is picked
and the user chooses to skip that commit then we need to remove that
commit's message from accumulated messages.  To do this 15ef69314d5
(rebase --skip: clean up commit message after a failed fixup/squash,
2018-04-27) updated commit_staged_changes() to reset the accumulated
message to the commit message of HEAD (which does not contain the
message from the skipped commit) when the last command was "fixup" or
"squash" and there are no staged changes. Unfortunately the code to do
this contains two bugs.

(1) If parse_head() fails we pass an invalid pointer to
    unuse_commit_buffer().

(2) The reconstructed message uses the entire commit buffer from HEAD
    including the headers, rather than just the commit message.

The first issue is fixed by splitting up the "if" condition into several
statements each with its own error handling. The second issue is fixed
by finding the start of the commit message within the commit buffer
using find_commit_subject().

The existing test added by 15ef69314d5 is modified to show the effect of
this bug.  The bug is triggered when skipping the first command in the
chain (as the test does before this commit) but the effect is hidden
because opts->current_fixup_count is set to zero which leads
update_squash_messages() to recreate the squash message file from
scratch overwriting the bad message created by
commit_staged_changes(). The test is also updated to explicitly check
the commit messages rather than relying on grep to ensure they do not
contain any stray commit headers.

To check the commit message the function test_commit_message() is moved
from t3437-rebase-fixup-options.sh to test-lib.sh. As the function is
now publicly available it is updated to provide better error detection
and avoid overwriting the commonly used files "actual" and "expect".
Support for reading the expected commit message from stdin is also
added.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>