builtin/bugreport.c: use thread-safe localtime_r()

builtin/bugreport.c: use thread-safe localtime_r()

To generate its filename, the 'git bugreport' builtin asks the system
for the current time with 'localtime()'. Since this uses a shared
buffer, it is not thread-safe.

Even though 'git bugreport' is not multi-threaded, using localtime() can
trigger some static analysis tools to complain, and a quick

    $ git grep -oh 'localtime\(_.\)\?' -- **/*.c | sort | uniq -c

shows that the only usage of the thread-unsafe 'localtime' is in a piece
of documentation.

So, convert this instance to use the thread-safe version for
consistency, and to appease some analysis tools.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>