Introduce is_bare_repository() and core.bare configuration variable

Introduce is_bare_repository() and core.bare configuration variable

This removes the old is_bare_git_dir(const char *) to ask if a
directory, if it is a GIT_DIR, is a bare repository, and
replaces it with is_bare_repository(void *).  The function looks
at core.bare configuration variable if exists but uses the old
heuristics: if it is ".git" or ends with "/.git", then it does
not look like a bare repository, otherwise it does.

Signed-off-by: Junio C Hamano <junkio@cox.net>