setup: document check_repository_format()

This function's interface is rather enigmatic, so let's
document it further.

While we're here, let's also drop the return value. It will
always either be "0" or the function will die (consequently,
neither of its two callers bothered to check the return).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -747,7 +747,14 @@ extern int grafts_replace_parents;
 #define GIT_REPO_VERSION_READ 1
 extern int repository_format_version;
 extern int repository_format_precious_objects;
-extern int check_repository_format(void);
+
+/*
+ * Check the repository format version in the path found in get_git_dir(),
+ * and die if it is a version we don't understand. Generally one would
+ * set_git_dir() before calling this, and use it only for "are we in a valid
+ * repo?".
+ */
+extern void check_repository_format(void);
 
 #define MTIME_CHANGED	0x0001
 #define CTIME_CHANGED	0x0002

setup.c

@@ -982,9 +982,9 @@ int check_repository_format_version(const char *var, const char *value, void *cb
 	return 0;
 }
 
-int check_repository_format(void)
+void check_repository_format(void)
 {
-	return check_repository_format_gently(get_git_dir(), NULL);
+	check_repository_format_gently(get_git_dir(), NULL);
 }
 
 /*