entry: extract a header file for entry.c functions

entry: extract a header file for entry.c functions

The declarations of entry.c's public functions and structures currently
reside in cache.h. Although not many, they contribute to the size of
cache.h and, when changed, cause the unnecessary recompilation of
modules that don't really use these functions. So let's move them to a
new entry.h header. While at it let's also move a comment related to
checkout_entry() from entry.c to entry.h as it's more useful to describe
the function there.

Original-patch-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>