add have_git_dir() function

This function is used to learn whether git_dir is already set up or not.
It is necessary, because we want to read configuration in compat/cygwin.c

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

cache.h

@@ -313,6 +313,7 @@ extern int is_bare_repository(void);
 extern int is_inside_git_dir(void);
 extern char *git_work_tree_cfg;
 extern int is_inside_work_tree(void);
+extern int have_git_dir(void);
 extern const char *get_git_dir(void);
 extern char *get_object_directory(void);
 extern char *get_index_file(void);

environment.c

@@ -80,6 +80,11 @@ int is_bare_repository(void)
 	return is_bare_repository_cfg && !get_git_work_tree();
 }
 
+int have_git_dir(void)
+{
+	return !!git_dir;
+}
+
 const char *get_git_dir(void)
 {
 	if (!git_dir)