Fix bogus comparison in load_options

This fixes the following warning from cppcheck:

   src/options.c:857:71: warning: comparison of constant '-1' with boolean expression is always false [-Wbool-compare]

Fixes #322
Fix a couple of pedantic warnings

This addresses the following type warnings emitted by clang:

 1) src/watch.c:192:2: warning: void function 'watch_apply' should not return void expression [-Wpedantic]
        return watch_apply_changes(source, WATCH_EVENT_LOAD, changed);
        ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 2) include/tig/keys.h:57:6: warning: ISO C forbids forward references to 'enum' types [-Wpedantic]
    enum status_code get_key_value(const char **name, struct key *key);
         ^

The warnings was emitted when executing the following command:

 > make clean all CFLAGS="-Wpedantic -Wno-extra-semi -Wno-variadic-macros -Wno-gnu-empty-initializer"
Add test for segfault related to TIG_SCRIPT usage