hgk: Display branch name for each head (issue 740)

In the graphical view, each head is decorated with an additonal tag
containing the branch name, if that head is not on the default branch.

contrib/hgk

@@ -370,6 +370,25 b' proc readrefs {} {'
 	lappend tagids($direct) $tag
 	lappend idtags($tag) $direct
     }
+
+    set status [catch {exec $env(HG) --config ui.report_untrusted=false heads} heads]
+    if { $status != 0 } {
+        puts $::errorInfo
+        if { ![string equal $::errorCode NONE] } {
+            exit 2
+        }
+    }
+    regsub -all "\r\n" $heads "\n" heads
+
+    set lines [split $heads "\n"]
+    foreach f $lines {
+        set match ""
+        regexp {changeset:\s+(\S+):(\S+)$} $f match id sha
+        if {$match != ""} {
+        lappend idheads($sha) $id
+        }
+    }
+
 }
 
 proc readotherrefs {base dname excl} {
@@ -1045,7 +1064,7 b' proc drawcommitline {level} {'
 }
 
 proc drawtags {id x xt y1} {
-    global idtags idheads idotherrefs
+    global idtags idheads idotherrefs commitinfo
     global linespc lthickness
     global canv mainfont idline rowtextx
 
@@ -1057,8 +1076,11 b' proc drawtags {id x xt y1} {'
 	set ntags [llength $marks]
     }
     if {[info exists idheads($id)]} {
-	set marks [concat $marks $idheads($id)]
-	set nheads [llength $idheads($id)]
+	set headmark [lindex $commitinfo($id) 7]
+	if {$headmark ne "default"} {
+	    lappend marks $headmark
+	    set nheads 1
+	}
     }
     if {[info exists idotherrefs($id)]} {
 	set marks [concat $marks $idotherrefs($id)]