builtin/log: stop using globals for log config

We're using global variables to store the log configuration. Many of
these can be set both via the command line and via the config, and
depending on how they are being set, they may contain allocated strings.
This leads to hard-to-track memory ownership and memory leaks.

Refactor the code to instead use a `struct log_config` that is being
allocated on the stack. This allows us to more clearly scope the
variables, track memory ownership and ultimately release the memory.

This also prepares us for a change to `git_config_string()`, which will
be adapted to have a `char **` out parameter instead of `const char **`.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t