range-diff: handle unterminated lines in read_patches()

range-diff: handle unterminated lines in read_patches()

When parsing our buffer of output from git-log, we have a
find_end_of_line() helper that finds the next newline, and gives us the
number of bytes to move past it, or the size of the whole remaining
buffer if there is no newline.

But trying to handle both those cases leads to some oddities:

  - we try to overwrite the newline with NUL in the caller, by writing
    over line[len-1]. This is at best redundant, since the helper will
    already have done so if it saw a newline. But if it didn't see a
    newline, it's actively wrong; we'll overwrite the byte at the end of
    the (unterminated) line.

    We could solve this just dropping the extra NUL assignment in the
    caller and just letting the helper do the right thing. But...

  - if we see a "diff --git" line, we'll restore the newline on top of
    the NUL byte, so we can pass the string to parse_git_diff_header().
    But if there was no newline in the first place, we can't do this.
    There's no place to put it (the current code writes a newline
    over whatever byte we obliterated earlier). The best we can do is
    feed the complete remainder of the buffer to the function (which is,
    in fact, a string, by virtue of being a strbuf).

To solve this, the caller needs to know whether we actually found a
newline or not. We could modify find_end_of_line() to return that
information, but we can further observe that it has only one caller.
So let's just inline it in that caller.

Nobody seems to have noticed this case, probably because git-log would
never produce input that doesn't end with a newline. Arguably we could
just return an error as soon as we see that the output does not end in a
newline. But the code to do so actually ends up _longer_, mostly because
of the cleanup we have to do in handling the error.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>