RebaseCommand: fix commit message in "fixup" case

RebaseCommand: fix commit message in "fixup" case

JGit accumulated in MESSAGE_FIXUP commit messages of a fixup sequence,
just like it did in MESSAGE_SQUASH, and on the last step of a sequence
of fixups used that file, after stripping all comment lines, as the
commit message. That also stripped any lines from the original commit
message that happened to start with the comment character.

This is not how this is supposed to work. MESSAGE_FIXUP must contain
the original commit message of the base commit that is amended, and
the file contains the verbatim commit message for the final fixup.[1]

Change the implementation accordingly, and add new tests.

[1] https://github.com/git/git/blob/df3c41adeb/sequencer.c#L86 ff.

Bug: 513726
Change-Id: I885a2b7f10d6c74460a8693aa6cbf867ee0494a1
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>