sequencer: introduce a helper to read files written by scripts

sequencer: introduce a helper to read files written by scripts

As we are slowly teaching the sequencer to perform the hard work for
the interactive rebase, we need to read files that were written by
shell scripts.

These files typically contain a single line and are invariably ended
by a line feed (and possibly a carriage return before that). Let's use
a helper to read such files and to remove the line ending.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>