branch --list: print useful info whilst interactive rebasing a detached HEAD

When rebasing interactively (rebase -i), "git branch --list" prints
a line indicating the current branch being rebased. This works well
when the interactive rebase is initiated when a local branch is
checked out.

This doesn't play well when the rebase is initiated on a detached
HEAD. When "git branch --list" tries to print information related
to the interactive rebase in this case it tries to print the name
of a branch using an uninitialized variable and thus tries to
print a "null pointer string". As a consequence, it does not provide
useful information while also inducing undefined behaviour.

So, print the point from which the rebase was started when interactive
rebasing a detached HEAD.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -1309,10 +1309,14 @@ char *get_head_description(void)
 	memset(&state, 0, sizeof(state));
 	wt_status_get_state(&state, 1);
 	if (state.rebase_in_progress ||
-	    state.rebase_interactive_in_progress)
-		strbuf_addf(&desc, _("(no branch, rebasing %s)"),
-			    state.branch);
-	else if (state.bisect_in_progress)
+	    state.rebase_interactive_in_progress) {
+		if (state.branch)
+			strbuf_addf(&desc, _("(no branch, rebasing %s)"),
+				    state.branch);
+		else
+			strbuf_addf(&desc, _("(no branch, rebasing detached HEAD %s)"),
+				    state.detached_from);
+	} else if (state.bisect_in_progress)
 		strbuf_addf(&desc, _("(no branch, bisect started on %s)"),
 			    state.branch);
 	else if (state.detached_from) {