Add API to update pr headBranch (#12419)

* [API] Add update pr headBranch api

Signed-off-by: a1012112796 <1012112796@qq.com>

routers/repo/pull.go

@@ -666,7 +666,7 @@ func ViewPullFiles(ctx *context.Context) {
 	ctx.HTML(200, tplPullFiles)
 }
 
-// UpdatePullRequest merge master into PR
+// UpdatePullRequest merge PR's baseBranch into headBranch
 func UpdatePullRequest(ctx *context.Context) {
 	issue := checkPullInfo(ctx)
 	if ctx.Written() {