Jeff King [off-list ref] writes:

I think it may be OK because true absolute paths imply that the first
entry would start with "/", and we would already have bailed earlier in
the function. So:

  diff --git /foo /bar

will already be rejected at the start of "/foo". And in broken input
like:

  diff --git a/foo /bar

we must assume that the start of "/bar" is a possible name, which is
what your patch is fixing. And in broken mixed input like that, we would
fail to find a valid split point, and correctly return NULL.

Correct.

I guess these happen in practice with "/dev/null" as the left-hand side.
But there we'd never need the names from this line, since we'd have a
separate "deleted file mode ..." header line.

Correct.  Besides, /dev/null appears on the ---/+++ line but not on
the "diff --git" line that is being parsed here.