[FEAT] Show follow symlink button

- When a user goes opens a symlink file in Forgejo, the file would be
rendered with the path of the symlink as content.
- Add a button that is shown when the user opens a *valid* symlink file,
which means that the symlink must have an valid path to an existent
file and after 999 follows isn't a symlink anymore.
- Return the relative path from the `FollowLink` functions, because Git
really doesn't want to tell where an file is located based on the blob ID.
- Adds integration tests.

routers/web/repo/view.go

@@ -114,7 +114,7 @@ func findReadmeFileInEntries(ctx *context.Context, entries []*git.TreeEntry, try
 			log.Debug("Potential readme file: %s", entry.Name())
 			if readmeFiles[i] == nil || base.NaturalSortLess(readmeFiles[i].Name(), entry.Blob().Name()) {
 				if entry.IsLink() {
-					target, err := entry.FollowLinks()
+					target, _, err := entry.FollowLinks()
 					if err != nil && !git.IsErrBadLink(err) {
 						return "", nil, err
 					} else if target != nil && (target.IsExecutable() || target.IsRegular()) {
@@ -267,7 +267,7 @@ func getFileReader(ctx gocontext.Context, repoID int64, blob *git.Blob) ([]byte,
 func renderReadmeFile(ctx *context.Context, subfolder string, readmeFile *git.TreeEntry) {
 	target := readmeFile
 	if readmeFile != nil && readmeFile.IsLink() {
-		target, _ = readmeFile.FollowLinks()
+		target, _, _ = readmeFile.FollowLinks()
 	}
 	if target == nil {
 		// if findReadmeFile() failed and/or gave us a broken symlink (which it shouldn't)
@@ -391,6 +391,15 @@ func renderFile(ctx *context.Context, entry *git.TreeEntry) {
 	ctx.Data["FileName"] = blob.Name()
 	ctx.Data["RawFileLink"] = ctx.Repo.RepoLink + "/raw/" + ctx.Repo.BranchNameSubURL() + "/" + util.PathEscapeSegments(ctx.Repo.TreePath)
 
+	if entry.IsLink() {
+		_, link, err := entry.FollowLinks()
+		// Errors should be allowed, because this shouldn't
+		// block rendering invalid symlink files.
+		if err == nil {
+			ctx.Data["SymlinkURL"] = ctx.Repo.RepoLink + "/src/" + ctx.Repo.BranchNameSubURL() + "/" + util.PathEscapeSegments(link)
+		}
+	}
+
 	commit, err := ctx.Repo.Commit.GetCommitByPath(ctx.Repo.TreePath)
 	if err != nil {
 		ctx.ServerError("GetCommitByPath", err)