Show number of commits a branch is ahead or behind the default branch

options/locale/locale_en-US.ini

@@ -898,6 +898,11 @@ commits.signed_by = Signed by
 commits.signed_by_untrusted_user = Signed by untrusted user
 commits.signed_by_untrusted_user_unmatched = Signed by untrusted user who does not match committer
 commits.gpg_key_id = GPG Key ID
+commits.count.ahead_1 = This branch is 1 commit ahead of %s.
+commits.count.behind_1 = This branch is 1 commit behind %s.
+commits.count.ahead_n = This branch is %d commits ahead of %s.
+commits.count.behind_n = This branch is %d commits behind %s.
+commits.count.even = This branch is even with %s.
 
 ext_issues = Ext. Issues
 ext_issues.desc = Link to an external issue tracker.

routers/repo/view.go

@@ -26,6 +26,7 @@ import (
 	"code.gitea.io/gitea/modules/lfs"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/repofiles"
 	"code.gitea.io/gitea/modules/setting"
 )
 
@@ -677,6 +678,40 @@ func renderCode(ctx *context.Context) {
 	ctx.Data["TreeLink"] = treeLink
 	ctx.Data["TreeNames"] = treeNames
 	ctx.Data["BranchLink"] = branchLink
+
+	if ctx.Repo.BranchName != ctx.Repo.Repository.DefaultBranch {
+
+		divergence, err := repofiles.CountDivergingCommits(ctx.Repo.Repository, ctx.Repo.BranchName)
+		if err != nil {
+			ctx.ServerError("CountDivergingCommits", err)
+			return
+		}
+
+		var msg string
+
+		if divergence.Ahead == 0 && divergence.Behind == 0 {
+			msg = ctx.Tr("repo.commits.count.even", ctx.Repo.Repository.DefaultBranch)
+		}
+
+		if divergence.Ahead > 0 {
+			if divergence.Ahead == 1 {
+				msg = ctx.Tr("repo.commits.count.ahead_1", ctx.Repo.Repository.DefaultBranch)
+			} else {
+				msg = ctx.Tr("repo.commits.count.ahead_n", divergence.Ahead, ctx.Repo.Repository.DefaultBranch)
+			}
+		}
+
+		if divergence.Behind > 0 {
+			if divergence.Behind == 1 {
+				msg = ctx.Tr("repo.commits.count.behind_1", ctx.Repo.Repository.DefaultBranch)
+			} else {
+				msg = ctx.Tr("repo.commits.count.behind_n", divergence.Behind, ctx.Repo.Repository.DefaultBranch)
+			}
+		}
+
+		ctx.Flash.Info(msg)
+	}
+
 	ctx.HTML(200, tplRepoHome)
 }