test-hashmap: squelch gcc compiler warning

test-hashmap: squelch gcc compiler warning

At least on this developer's MacOSX (Snow Leopard, gcc-4.2.1), GCC
prints a warning that 'hash' may be used uninitialized when
compiling test-hashmap that 'hash' may be used uninitialized (but
GCC 4.6.3 on this developer's Ubuntu server does not report this
problem).

The old compiler is wrong, of course, as the switch (method & 3)
statement already handles all the possible cases, but that does not
help in a scenario where it is hard or impossible to upgrade to a
newer compiler (e.g. being stuck on an older MacOSX and having to
rely on Xcode).

So let's just initialize the variable and be done with it, it is
hardly a crucial part of the code because it is only used by the
test suite and invisible to the end users.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>