gitk: Allow forcing branch creation if it already exists

If gitk knows that the branch the user tries to create exists,
it should ask whether it should overwrite it.  This way the user
can either decide to choose a new name, or move the head while
preserving the reflog.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -7727,24 +7727,42 @@ proc mkbrgo {top} {
 
     set name [$top.name get]
     set id [$top.sha1 get]
+    set cmdargs {}
+    set old_id {}
     if {$name eq {}} {
 	error_popup [mc "Please specify a name for the new branch"]
 	return
     }
+    if {[info exists headids($name)]} {
+	if {![confirm_popup [mc \
+		"Branch '%s' already exists. Overwrite?" $name]]} {
+	    return
+	}
+	set old_id $headids($name)
+	lappend cmdargs -f
+    }
     catch {destroy $top}
+    lappend cmdargs $name $id
     nowbusy newbranch
     update
     if {[catch {
-	exec git branch $name $id
+	eval exec git branch $cmdargs
     } err]} {
 	notbusy newbranch
 	error_popup $err
     } else {
-	set headids($name) $id
-	lappend idheads($id) $name
-	addedhead $id $name
 	notbusy newbranch
-	redrawtags $id
+	if {$old_id ne {}} {
+	    movehead $id $name
+	    movedhead $id $name
+	    redrawtags $old_id
+	    redrawtags $id
+	} else {
+	    set headids($name) $id
+	    lappend idheads($id) $name
+	    addedhead $id $name
+	    redrawtags $id
+	}
 	dispneartags 0
 	run refill_reflist
     }