diff --git a/cache.h b/cache.h
index 160ff81aa7..82f39f7a9e 100644
--- a/cache.h
+++ b/cache.h
@@ -525,12 +525,15 @@ extern void set_git_work_tree(const char *tree);
 
 extern void setup_work_tree(void);
 /*
- * Find GIT_DIR of the repository that contains the current working directory,
- * without changing the working directory or other global state. The result is
- * appended to gitdir. The return value is either NULL if no repository was
- * found, or pointing to the path inside gitdir's buffer.
+ * Find the commondir and gitdir of the repository that contains the current
+ * working directory, without changing the working directory or other global
+ * state. The result is appended to commondir and gitdir.  If the discovered
+ * gitdir does not correspond to a worktree, then 'commondir' and 'gitdir' will
+ * both have the same result appended to the buffer.  The return value is
+ * either 0 upon success and non-zero if no repository was found.
  */
-extern const char *discover_git_directory(struct strbuf *gitdir);
+extern int discover_git_directory(struct strbuf *commondir,
+				  struct strbuf *gitdir);
 extern const char *setup_git_directory_gently(int *);
 extern const char *setup_git_directory(void);
 extern char *prefix_path(const char *prefix, int len, const char *path);
diff --git a/config.c b/config.c
index eec7613e04..e8dbf9e64a 100644
--- a/config.c
+++ b/config.c
@@ -1639,7 +1639,8 @@ static void configset_iter(struct config_set *cs, config_fn_t fn, void *data)
 void read_early_config(config_fn_t cb, void *data)
 {
 	struct config_options opts = {0};
-	struct strbuf buf = STRBUF_INIT;
+	struct strbuf commondir = STRBUF_INIT;
+	struct strbuf gitdir = STRBUF_INIT;
 
 	opts.respect_includes = 1;
 
@@ -1653,12 +1654,13 @@ void read_early_config(config_fn_t cb, void *data)
 	 * notably, the current working directory is still the same after the
 	 * call).
 	 */
-	else if (discover_git_directory(&buf))
-		opts.git_dir = buf.buf;
+	else if (!discover_git_directory(&commondir, &gitdir))
+		opts.git_dir = gitdir.buf;
 
 	git_config_with_options(cb, data, NULL, &opts);
 
-	strbuf_release(&buf);
+	strbuf_release(&commondir);
+	strbuf_release(&gitdir);
 }
 
 static void git_config_check_init(void);
diff --git a/setup.c b/setup.c
index c45443f696..4e38cf2a4f 100644
--- a/setup.c
+++ b/setup.c
@@ -941,19 +941,21 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 	}
 }
 
-const char *discover_git_directory(struct strbuf *gitdir)
+int discover_git_directory(struct strbuf *commondir,
+			   struct strbuf *gitdir)
 {
 	struct strbuf dir = STRBUF_INIT, err = STRBUF_INIT;
 	size_t gitdir_offset = gitdir->len, cwd_len;
+	size_t commondir_offset = commondir->len;
 	struct repository_format candidate;
 
 	if (strbuf_getcwd(&dir))
-		return NULL;
+		return -1;
 
 	cwd_len = dir.len;
 	if (setup_git_directory_gently_1(&dir, gitdir, 0) <= 0) {
 		strbuf_release(&dir);
-		return NULL;
+		return -1;
 	}
 
 	/*
@@ -969,8 +971,10 @@ const char *discover_git_directory(struct strbuf *gitdir)
 		strbuf_insert(gitdir, gitdir_offset, dir.buf, dir.len);
 	}
 
+	get_common_dir(commondir, gitdir->buf + gitdir_offset);
+
 	strbuf_reset(&dir);
-	strbuf_addf(&dir, "%s/config", gitdir->buf + gitdir_offset);
+	strbuf_addf(&dir, "%s/config", commondir->buf + commondir_offset);
 	read_repository_format(&candidate, dir.buf);
 	strbuf_release(&dir);
 
@@ -978,11 +982,12 @@ const char *discover_git_directory(struct strbuf *gitdir)
 		warning("ignoring git dir '%s': %s",
 			gitdir->buf + gitdir_offset, err.buf);
 		strbuf_release(&err);
+		strbuf_setlen(commondir, commondir_offset);
 		strbuf_setlen(gitdir, gitdir_offset);
-		return NULL;
+		return -1;
 	}
 
-	return gitdir->buf + gitdir_offset;
+	return 0;
 }
 
 const char *setup_git_directory_gently(int *nongit_ok)