environment: move set_git_dir() and related into setup layer

The functions `set_git_dir()` and friends are used to set up
repositories. As such, they are quite clearly part of the setup
subsystem, but still live in "environment.c". Move them over, which also
helps to get rid of dependencies on `the_repository` in the environment
subsystem.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.h

@@ -94,6 +94,9 @@ static inline int discover_git_directory(struct strbuf *commondir,
 	return 0;
 }
 
+void set_git_dir(const char *path, int make_realpath);
+void set_git_work_tree(const char *tree);
+
 const char *setup_git_directory_gently(int *);
 const char *setup_git_directory(void);
 char *prefix_path(const char *prefix, int len, const char *path);