Fix off-by-one error in RebaseTodoFile when reading a todo file

Fix off-by-one error in RebaseTodoFile when reading a todo file

Commit messages of length 1 were not read. 'lineEnd' is the offset
of the last character in the line before the terminating LF or CR-LF,
and 'nextSpace' is actually the offset of the character _after_ the
next space. With a one-character commit message, nextSpace == lineEnd.

The code also assumes the commit message to be optional, but actually
failed in that case because it read beyond the line ending. Fix that,
too.

Add a test case for reading a todo file.

Bug: 546245
Change-Id: I368d63615930ea2398a6230e756442fd88870654
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>