git wrapper: introduce startup_info struct

The startup_info struct will collect information managed by the git
setup code, such as the prefix for relative paths passed on the
command line (i.e., path to the starting cwd from the toplevel of
the work tree) and whether a git repository has been found.

In other words, startup_info is intended to be a collection of global
variables with results that were previously returned from setup
functions.  This state is global anyway (since the cwd is), even
if it is not currently tracked that way.  Letting these values persist
means there is more flexibility in deciding when to run setup.

For now, the struct is empty.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1097,6 +1097,11 @@ void overlay_tree_on_cache(const char *tree_name, const char *prefix);
 char *alias_lookup(const char *alias);
 int split_cmdline(char *cmdline, const char ***argv);
 
+/* git.c */
+struct startup_info {
+};
+extern struct startup_info *startup_info;
+
 /* builtin/merge.c */
 int checkout_fast_forward(const unsigned char *from, const unsigned char *to);