setup: allow for prefix to be passed to git commands

In a future patch child processes which act on submodules need a little
more context about the original command that was invoked.  This patch
teaches git to use the prefix stored in `GIT_INTERNAL_TOPLEVEL_PREFIX`
instead of the prefix that was potentally found during the git directory
setup process.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -410,6 +410,7 @@ static inline enum object_type object_type(unsigned int mode)
 #define GIT_WORK_TREE_ENVIRONMENT "GIT_WORK_TREE"
 #define GIT_PREFIX_ENVIRONMENT "GIT_PREFIX"
 #define GIT_SUPER_PREFIX_ENVIRONMENT "GIT_INTERNAL_SUPER_PREFIX"
+#define GIT_TOPLEVEL_PREFIX_ENVIRONMENT "GIT_INTERNAL_TOPLEVEL_PREFIX"
 #define DEFAULT_GIT_DIR_ENVIRONMENT ".git"
 #define DB_ENVIRONMENT "GIT_OBJECT_DIRECTORY"
 #define INDEX_ENVIRONMENT "GIT_INDEX_FILE"

git.c

@@ -361,8 +361,6 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 	if (!help && get_super_prefix()) {
 		if (!(p->option & SUPPORT_SUPER_PREFIX))
 			die("%s doesn't support --super-prefix", p->cmd);
-		if (prefix)
-			die("can't use --super-prefix from a subdirectory");
 	}
 
 	if (!help && p->option & NEED_WORK_TREE)

setup.c

@@ -939,9 +939,14 @@ static const char *setup_git_directory_gently_1(int *nongit_ok)
 
 const char *setup_git_directory_gently(int *nongit_ok)
 {
-	const char *prefix;
+	const char *prefix, *env_prefix;
 
 	prefix = setup_git_directory_gently_1(nongit_ok);
+	env_prefix = getenv(GIT_TOPLEVEL_PREFIX_ENVIRONMENT);
+
+	if (env_prefix)
+		prefix = env_prefix;
+
 	if (prefix)
 		setenv(GIT_PREFIX_ENVIRONMENT, prefix, 1);
 	else