Add bitbucket forge BranchHead implementation (#2011)

In order to test this functionality, we had to expose the `BranchHead()`
through an API endpoint
```
GET /repos/{repo_id}/branches/{branch}/head
```

The response is a string that contains the latest commit hash of the
requested branch.

server/forge/bitbucket/bitbucket.go

@@ -297,9 +297,8 @@ func (c *config) Branches(ctx context.Context, u *model.User, r *model.Repo, _ *
 }
 
 // BranchHead returns the sha of the head (latest commit) of the specified branch
-func (c *config) BranchHead(_ context.Context, _ *model.User, _ *model.Repo, _ string) (string, error) {
-	// TODO(1138): missing implementation
-	return "", forge_types.ErrNotImplemented
+func (c *config) BranchHead(ctx context.Context, u *model.User, r *model.Repo, branch string) (string, error) {
+	return c.newClient(ctx, u).GetBranchHead(r.Owner, r.Name, branch)
 }
 
 // PullRequests returns the pull requests of the named repository.

server/forge/bitbucket/internal/client.go

@@ -47,6 +47,7 @@ const (
 	pathBranches      = "%s/2.0/repositories/%s/%s/refs/branches"
 	pathOrgPerms      = "%s/2.0/workspaces/%s/permissions?%s"
 	pathPullRequests  = "%s/2.0/repositories/%s/%s/pullrequests"
+	pathBranchCommits = "%s/2.0/repositories/%s/%s/commits/%s"
 )
 
 type Client struct {
@@ -183,6 +184,19 @@ func (c *Client) ListBranches(owner, name string) ([]*Branch, error) {
 	return out.Values, err
 }
 
+func (c *Client) GetBranchHead(owner, name, branch string) (string, error) {
+	out := new(CommitsResp)
+	uri := fmt.Sprintf(pathBranchCommits, c.base, owner, name, branch)
+	_, err := c.do(uri, get, nil, out)
+	if err != nil {
+		return "", err
+	}
+	if len(out.Values) == 0 {
+		return "", fmt.Errorf("no commits in branch %s", branch)
+	}
+	return out.Values[0].Hash, nil
+}
+
 func (c *Client) GetUserWorkspaceMembership(workspace, user string) (string, error) {
 	out := new(WorkspaceMembershipResp)
 	opts := &ListOpts{Page: 1, PageLen: 100}

server/forge/bitbucket/internal/types.go

@@ -271,3 +271,11 @@ type PullRequest struct {
 	ID    uint   `json:"id"`
 	Title string `json:"title"`
 }
+
+type CommitsResp struct {
+	Values []*Commit `json:"values"`
+}
+
+type Commit struct {
+	Hash string `json:"hash"`
+}