branch: show more information when HEAD is detached

This prints more helpful info when HEAD is detached: is it detached
because of bisect or rebase? What is the original branch name in those
cases? Is it detached because the user checks out a remote ref or a
tag (and which one)?

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -18,6 +18,7 @@
 #include "string-list.h"
 #include "column.h"
 #include "utf8.h"
+#include "wt-status.h"
 
 static const char * const builtin_branch_usage[] = {
 	N_("git branch [options] [-r | -a] [--merged | --no-merged]"),
@@ -550,6 +551,29 @@ static int calc_maxwidth(struct ref_list *refs)
 	return w;
 }
 
+static char *get_head_description(void)
+{
+	struct strbuf desc = STRBUF_INIT;
+	struct wt_status_state state;
+	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)
+		strbuf_addf(&desc, _("(no branch, bisecting %s)"),
+			    state.branch);
+	else if (state.detached_from)
+		strbuf_addf(&desc, _("(detached from %s)"),
+			    state.detached_from);
+	else
+		strbuf_addstr(&desc, _("(no branch)"));
+	free(state.branch);
+	free(state.onto);
+	free(state.detached_from);
+	return strbuf_detach(&desc, NULL);
+}
 
 static void show_detached(struct ref_list *ref_list)
 {
@@ -557,7 +581,7 @@ static void show_detached(struct ref_list *ref_list)
 
 	if (head_commit && is_descendant_of(head_commit, ref_list->with_commit)) {
 		struct ref_item item;
-		item.name = xstrdup(_("(no branch)"));
+		item.name = get_head_description();
 		item.width = utf8_strwidth(item.name);
 		item.kind = REF_LOCAL_BRANCH;
 		item.dest = NULL;

t/t3203-branch-output.sh

@@ -94,13 +94,13 @@ test_expect_success 'git branch -v pattern does not show branch summaries' '
 	test_must_fail git branch -v branch*
 '
 
-cat >expect <<'EOF'
-* (no branch)
+test_expect_success 'git branch shows detached HEAD properly' '
+	cat >expect <<EOF &&
+* (detached from $(git rev-parse --short HEAD^0))
   branch-one
   branch-two
   master
 EOF
-test_expect_success 'git branch shows detached HEAD properly' '
 	git checkout HEAD^0 &&
 	git branch >actual &&
 	test_i18ncmp expect actual

t/t6030-bisect-porcelain.sh

@@ -164,7 +164,7 @@ test_expect_success 'bisect start: existing ".git/BISECT_START" not modified if
 	cp .git/BISECT_START saved &&
 	test_must_fail git bisect start $HASH4 foo -- &&
 	git branch > branch.output &&
-	test_i18ngrep "* (no branch)" branch.output > /dev/null &&
+	test_i18ngrep "* (no branch, bisecting other)" branch.output > /dev/null &&
 	test_cmp saved .git/BISECT_START
 '
 test_expect_success 'bisect start: no ".git/BISECT_START" if mistaken rev' '