Fix bug when visiting comparation page (#34334) (#34365)

Backport #34334 by @lunny

The `ci.HeadGitRepo` was opened and closed in the function
`ParseCompareInfo` but reused in the function `PrepareCompareDiff`.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/pull.go

@@ -1290,11 +1290,6 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	)
 
 	ci := ParseCompareInfo(ctx)
-	defer func() {
-		if ci != nil && ci.HeadGitRepo != nil {
-			ci.HeadGitRepo.Close()
-		}
-	}()
 	if ctx.Written() {
 		return
 	}