log: decorate HEAD with branch name

Currently, log decorations do not indicate which branch is checked out
and whether HEAD is detached.

When branch foo is checked out, change the "HEAD, foo" part of the
decorations to "HEAD -> foo". This serves to indicate both ref
decorations (helped by the spacing) as well as their relationshsip.
As a consequence, "HEAD" without any " -> " denotes a detached HEAD now.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

diff --git a/log-tree.c b/log-tree.c
index 53bb526..2c1ed0f 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -173,6 +173,43 @@
 }
 
 /*
+ * Do we have HEAD in the output, and also the branch it points at?
+ * If so, find that decoration entry for that current branch.
+ */
+static const struct name_decoration *current_pointed_by_HEAD(const struct name_decoration *decoration)
+{
+	const struct name_decoration *list, *head = NULL;
+	const char *branch_name = NULL;
+	unsigned char unused[20];
+	int rru_flags;
+
+	/* First find HEAD */
+	for (list = decoration; list; list = list->next)
+		if (list->type == DECORATION_REF_HEAD) {
+			head = list;
+			break;
+		}
+	if (!head)
+		return NULL;
+
+	/* Now resolve and find the matching current branch */
+	branch_name = resolve_ref_unsafe("HEAD", 0, unused, &rru_flags);
+	if (!(rru_flags & REF_ISSYMREF))
+		return NULL;
+	if (!skip_prefix(branch_name, "refs/heads/", &branch_name))
+		return NULL;
+
+	/* OK, do we have that ref in the list? */
+	for (list = decoration; list; list = list->next)
+		if ((list->type == DECORATION_REF_LOCAL) &&
+		    !strcmp(branch_name, list->name)) {
+			return list;
+		}
+
+	return NULL;
+}
+
+/*
  * The caller makes sure there is no funny color before calling.
  * format_decorations_extended makes sure the same after return.
  */
@@ -184,6 +221,7 @@
 			const char *suffix)
 {
 	const struct name_decoration *decoration;
+	const struct name_decoration *current_and_HEAD;
 	const char *color_commit =
 		diff_get_color(use_color, DIFF_COMMIT);
 	const char *color_reset =
@@ -192,16 +230,37 @@
 	decoration = get_name_decoration(&commit->object);
 	if (!decoration)
 		return;
+
+	current_and_HEAD = current_pointed_by_HEAD(decoration);
 	while (decoration) {
-		strbuf_addstr(sb, color_commit);
-		strbuf_addstr(sb, prefix);
-		strbuf_addstr(sb, color_reset);
-		strbuf_addstr(sb, decorate_get_color(use_color, decoration->type));
-		if (decoration->type == DECORATION_REF_TAG)
-			strbuf_addstr(sb, "tag: ");
-		strbuf_addstr(sb, decoration->name);
-		strbuf_addstr(sb, color_reset);
-		prefix = separator;
+		/*
+		 * When both current and HEAD are there, only
+		 * show HEAD->current where HEAD would have
+		 * appeared, skipping the entry for current.
+		 */
+		if (decoration != current_and_HEAD) {
+			strbuf_addstr(sb, color_commit);
+			strbuf_addstr(sb, prefix);
+			strbuf_addstr(sb, color_reset);
+			strbuf_addstr(sb, decorate_get_color(use_color, decoration->type));
+			if (decoration->type == DECORATION_REF_TAG)
+				strbuf_addstr(sb, "tag: ");
+
+			strbuf_addstr(sb, decoration->name);
+
+			if (current_and_HEAD &&
+			    decoration->type == DECORATION_REF_HEAD) {
+				strbuf_addstr(sb, color_reset);
+				strbuf_addstr(sb, color_commit);
+				strbuf_addstr(sb, " -> ");
+				strbuf_addstr(sb, color_reset);
+				strbuf_addstr(sb, decorate_get_color(use_color, current_and_HEAD->type));
+				strbuf_addstr(sb, current_and_HEAD->name);
+			}
+			strbuf_addstr(sb, color_reset);
+
+			prefix = separator;
+		}
 		decoration = decoration->next;
 	}
 	strbuf_addstr(sb, color_commit);

t/t4013/diff.log_--decorate_--all

diff --git a/t/t4013/diff.log_--decorate_--all b/t/t4013/diff.log_--decorate_--all
index 27d3eab..3aa16a9 100644
--- a/t/t4013/diff.log_--decorate_--all
+++ b/t/t4013/diff.log_--decorate_--all
@@ -5,7 +5,7 @@
 
     Rearranged lines in dir/sub
 
-commit 59d314ad6f356dd08601a4cd5e530381da3e3c64 (HEAD, master)
+commit 59d314ad6f356dd08601a4cd5e530381da3e3c64 (HEAD -> master)
 Merge: 9a6d494 c7a2ab9
 Author: A U Thor <author@example.com>
 Date:   Mon Jun 26 00:04:00 2006 +0000

t/t4207-log-decoration-colors.sh

diff --git a/t/t4207-log-decoration-colors.sh b/t/t4207-log-decoration-colors.sh
index 6b8ad4f..f8008b6 100755
--- a/t/t4207-log-decoration-colors.sh
+++ b/t/t4207-log-decoration-colors.sh
@@ -44,10 +44,10 @@
 '
 
 cat >expected <<EOF
-${c_commit}COMMIT_ID${c_reset}${c_commit} (${c_reset}${c_HEAD}HEAD${c_reset}${c_commit},\
+${c_commit}COMMIT_ID${c_reset}${c_commit} (${c_reset}${c_HEAD}HEAD${c_reset}${c_commit} ->\
+ ${c_reset}${c_branch}master${c_reset}${c_commit},\
  ${c_reset}${c_tag}tag: v1.0${c_reset}${c_commit},\
- ${c_reset}${c_tag}tag: B${c_reset}${c_commit},\
- ${c_reset}${c_branch}master${c_reset}${c_commit})${c_reset} B
+ ${c_reset}${c_tag}tag: B${c_reset}${c_commit})${c_reset} B
 ${c_commit}COMMIT_ID${c_reset}${c_commit} (${c_reset}${c_tag}tag: A1${c_reset}${c_commit},\
  ${c_reset}${c_remoteBranch}other/master${c_reset}${c_commit})${c_reset} A1
 ${c_commit}COMMIT_ID${c_reset}${c_commit} (${c_reset}${c_stash}refs/stash${c_reset}${c_commit})${c_reset}\