name-rev: pull out deref handling from the recursion

name-rev: pull out deref handling from the recursion

The 'if (deref) { ... }' condition near the beginning of the recursive
name_rev() function can only ever be true in the first invocation,
because the 'deref' parameter is always 0 in the subsequent recursive
invocations.

Extract this condition from the recursion into name_rev()'s caller and
drop the function's 'deref' parameter.  This makes eliminating the
recursion a bit easier to follow, and it will be moved back into
name_rev() after the recursion is eliminated.

Furthermore, drop the condition that die()s when both 'deref' and
'generation' are non-null (which should have been a BUG() to begin
with).

Note that this change reintroduces the memory leak that was plugged in
in commit 5308224633 (name-rev: avoid leaking memory in the `deref`
case, 2017-05-04), but a later patch (name-rev: restructure
creating/updating 'struct rev_name' instances) in this series will
plug it in again.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>