dir: move starts_with_dot(_dot)_slash to dir.h

dir: move starts_with_dot(_dot)_slash to dir.h

Both submodule--helper.c and submodule-config.c have an implementation
of starts_with_dot_slash and starts_with_dot_dot_slash. The dir.h header
has starts_with_dot(_dot)_slash_native, which sets PATH_MATCH_NATIVE.

Move the helpers to dir.h as static inlines. I thought about renaming
them to postfix with _platform but that felt too long and ugly. On the
other hand it might be slightly confusing with _native.

This simplifies a submodule refactor which wants to use the helpers
earlier in the submodule--helper.c file.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>