The variables `git_config_escaped` and `git_config_escapes` are both
defined as static const character pointers in "config_parse.h". In case
where "config_parse.h" is included but those two variables are not being
used, the compiler will thus complain about defined but unused
variables. Fix this by declaring them as external and moving the actual
initialization to the C file.

Note that it is not possible to simply make this a #define, as we are
indexing into those arrays.