Fix an enum assignment issue spotted by Sun Studio

Fix an enum assignment issue spotted by Sun Studio

In builtin/fast-export.c we'd assign to variables of the
tag_of_filtered_mode enum type with constants defined for the
signed_tag_mode enum.

We'd get the intended value since both the value we were assigning
with and the one we actually wanted had the same positional within
their respective enums, but doing it this way makes no sense.

This issue was spotted by Sun Studio 12 Update 1:

    "builtin/fast-export.c", line 54: warning: enum type mismatch: op "=" (E_ENUM_TYPE_MISMATCH_OP)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>