mailmap: handle mailmap blobs without trailing newlines

The read_mailmap_buf function reads each line of the mailmap using strchrnul, like: const char *end = strchrnul(buf, '\n'); unsigned long linelen = end - buf + 1; But that's off-by-one when we actually hit the NUL byte; our line does not have a terminator, and so is only "end - buf" bytes long. As a result, when we subtract the linelen from the total len, we end up with (unsigned long)-1 bytes left in the buffer, and we start reading random junk from memory. We could fix it with: unsigned long linelen = end - buf + !!*end; but let's take a step back for a moment. It's questionable in the first place for a function that takes a buffer and length to be using strchrnul. But it works because we only have one caller (and are only likely to ever have this one), which is handing us data from read_sha1_file. Which means that it's always NUL-terminated. Instead of tightening the assumptions to make the buffer/length pair work for a caller that doesn't actually exist, let's let loosen the assumptions to what the real caller has: a modifiable, NUL-terminated string. This makes the code simpler and shorter (because we don't have to correlate strchrnul with the length calculation), correct (because the code with the off-by-one just goes away), and more efficient (we can drop the extra allocation we needed to create NUL-terminated strings for each line, and just terminate in place). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>