Restore detection of branches are equal on compare page (#14586)

Somehow the test for detecting if branches are equal broke
this PR restores this functionality.

Fix #14502

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/repo_compare.go

@@ -21,6 +21,8 @@ import (
 // CompareInfo represents needed information for comparing references.
 type CompareInfo struct {
 	MergeBase    string
+	BaseCommitID string
+	HeadCommitID string
 	Commits      *list.List
 	NumFiles     int
 }
@@ -66,8 +68,18 @@ func (repo *Repository) GetCompareInfo(basePath, baseBranch, headBranch string)
 	}
 
 	compareInfo := new(CompareInfo)
+
+	compareInfo.HeadCommitID, err = GetFullCommitID(repo.Path, headBranch)
+	if err != nil {
+		compareInfo.HeadCommitID = headBranch
+	}
+
 	compareInfo.MergeBase, remoteBranch, err = repo.GetMergeBase(tmpRemote, baseBranch, headBranch)
 	if err == nil {
+		compareInfo.BaseCommitID, err = GetFullCommitID(repo.Path, remoteBranch)
+		if err != nil {
+			compareInfo.BaseCommitID = remoteBranch
+		}
 		// We have a common base - therefore we know that ... should work
 		logs, err := NewCommand("log", compareInfo.MergeBase+"..."+headBranch, prettyLogFormat).RunInDirBytes(repo.Path)
 		if err != nil {
@@ -83,6 +95,7 @@ func (repo *Repository) GetCompareInfo(basePath, baseBranch, headBranch string)
 		if err != nil {
 			compareInfo.MergeBase = remoteBranch
 		}
+		compareInfo.BaseCommitID = compareInfo.MergeBase
 	}
 
 	// Count number of changed files.

routers/repo/compare.go

@@ -423,18 +423,7 @@ func PrepareCompareDiff(
 	// Get diff information.
 	ctx.Data["CommitRepoLink"] = headRepo.Link()
 
-	headCommitID := headBranch
+	headCommitID := compareInfo.HeadCommitID
-	if ctx.Data["HeadIsCommit"] == false {
-		if ctx.Data["HeadIsTag"] == true {
-			headCommitID, err = headGitRepo.GetTagCommitID(headBranch)
-		} else {
-			headCommitID, err = headGitRepo.GetBranchCommitID(headBranch)
-		}
-		if err != nil {
-			ctx.ServerError("GetRefCommitID", err)
-			return false
-		}
-	}
 
 	ctx.Data["AfterCommitID"] = headCommitID
 
@@ -460,18 +449,7 @@ func PrepareCompareDiff(
 	}
 
 	baseGitRepo := ctx.Repo.GitRepo
-	baseCommitID := baseBranch
+	baseCommitID := compareInfo.BaseCommitID
-	if ctx.Data["BaseIsCommit"] == false {
-		if ctx.Data["BaseIsTag"] == true {
-			baseCommitID, err = baseGitRepo.GetTagCommitID(baseBranch)
-		} else {
-			baseCommitID, err = baseGitRepo.GetBranchCommitID(baseBranch)
-		}
-		if err != nil {
-			ctx.ServerError("GetRefCommitID", err)
-			return false
-		}
-	}
 
 	baseCommit, err := baseGitRepo.GetCommit(baseCommitID)
 	if err != nil {