improve force push detect when push (#2798)

Author: lunny

cmd/hook.go

@@ -118,7 +118,7 @@ func runHookPreReceive(c *cli.Context) error {
 		if protectBranch != nil && protectBranch.IsProtected() {
 			// detect force push
 			if git.EmptySHA != oldCommitID {
-				output, err := git.NewCommand("rev-list", oldCommitID, "^"+newCommitID).RunInDir(repoPath)
+				output, err := git.NewCommand("rev-list", "--max-count=1", oldCommitID, "^"+newCommitID).RunInDir(repoPath)
 				if err != nil {
 					fail("Internal error", "Fail to detect force push: %v", err)
 				} else if len(output) > 0 {