Support standard path variable replacement syntax

Add support for the following synonyms:

{owner} for :owner
{repo} for :repo
{branch} for :branch

pkg/cmd/api/api.go

@@ -71,7 +71,7 @@ func NewCmdApi(f *cmdutil.Factory, runF func(*ApiOptions) error) *cobra.Command
 			The endpoint argument should either be a path of a GitHub API v3 endpoint, or
 			"graphql" to access the GitHub API v4.
 
-			Placeholder values ":owner", ":repo", and ":branch" in the endpoint argument will
+			Placeholder values "{owner}", "{repo}", and "{branch}" in the endpoint argument will
 			get replaced with values from the repository of the current directory.
 
 			The default HTTP request method is "GET" normally and "POST" if any parameters
@@ -87,7 +87,7 @@ func NewCmdApi(f *cmdutil.Factory, runF func(*ApiOptions) error) *cobra.Command
 
 			- literal values "true", "false", "null", and integer numbers get converted to
 			  appropriate JSON types;
-			- placeholder values ":owner", ":repo", and ":branch" get populated with values
+			- placeholder values "{owner}", "{repo}", and "{branch}" get populated with values
 			  from the repository of the current directory;
 			- if the value starts with "@", the rest of the value is interpreted as a
 			  filename to read the value from. Pass "-" to read from standard input.
@@ -106,10 +106,10 @@ func NewCmdApi(f *cmdutil.Factory, runF func(*ApiOptions) error) *cobra.Command
 		`, "`"),
 		Example: heredoc.Doc(`
 			# list releases in the current repository
-			$ gh api repos/:owner/:repo/releases
+			$ gh api repos/{owner}/{repo}/releases
 
 			# post an issue comment
-			$ gh api repos/:owner/:repo/issues/123/comments -f body='Hi from CLI'
+			$ gh api repos/{owner}/{repo}/issues/123/comments -f body='Hi from CLI'
 
 			# add parameters to a GET request
 			$ gh api -X GET search/issues -f q='repo:cli/cli is:open remote'
@@ -121,14 +121,14 @@ func NewCmdApi(f *cmdutil.Factory, runF func(*ApiOptions) error) *cobra.Command
 			$ gh api --preview baptiste,nebula ...
 
 			# print only specific fields from the response
-			$ gh api repos/:owner/:repo/issues --jq '.[].title'
+			$ gh api repos/{owner}/{repo}/issues --jq '.[].title'
 
 			# use a template for the output
-			$ gh api repos/:owner/:repo/issues --template \
+			$ gh api repos/{owner}/{repo}/issues --template \
 			  '{{range .}}{{.title}} ({{.labels | pluck "name" | join ", " | color "yellow"}}){{"\n"}}{{end}}'
 
 			# list releases with GraphQL
-			$ gh api graphql -F owner=':owner' -F name=':repo' -f query='
+			$ gh api graphql -F owner='{owner}' -F name='{repo}' -f query='
 			  query($name: String!, $owner: String!) {
 			    repository(owner: $owner, name: $name) {
 			      releases(last: 3) {
@@ -397,9 +397,9 @@ func processResponse(resp *http.Response, opts *ApiOptions, headersOutputStream
 	return
 }
 
-var placeholderRE = regexp.MustCompile(`\:(owner|repo|branch)\b`)
+var placeholderRE = regexp.MustCompile(`(\:(owner|repo|branch)\b|\{(owner|repo|branch)\})`)
 
-// fillPlaceholders populates `:owner` and `:repo` placeholders with values from the current repository
+// fillPlaceholders populates `{owner}` and `{repo}` placeholders with values from the current repository
 func fillPlaceholders(value string, opts *ApiOptions) (string, error) {
 	if !placeholderRE.MatchString(value) {
 		return value, nil
@@ -412,11 +412,11 @@ func fillPlaceholders(value string, opts *ApiOptions) (string, error) {
 
 	filled := placeholderRE.ReplaceAllStringFunc(value, func(m string) string {
 		switch m {
-		case ":owner":
+		case ":owner", "{owner}":
 			return baseRepo.RepoOwner()
-		case ":repo":
+		case ":repo", "{repo}":
 			return baseRepo.RepoName()
-		case ":branch":
+		case ":branch", "{branch}":
 			branch, e := opts.Branch()
 			if e != nil {
 				err = e

pkg/cmd/api/api_test.go

@@ -870,7 +870,7 @@ func Test_magicFieldValue(t *testing.T) {
 			wantErr: false,
 		},
 		{
-			name: "placeholder",
+			name: "placeholder colon",
 			args: args{
 				v: ":owner",
 				opts: &ApiOptions{
@@ -883,6 +883,20 @@ func Test_magicFieldValue(t *testing.T) {
 			want:    "hubot",
 			wantErr: false,
 		},
+		{
+			name: "placeholder braces",
+			args: args{
+				v: "{owner}",
+				opts: &ApiOptions{
+					IO: io,
+					BaseRepo: func() (ghrepo.Interface, error) {
+						return ghrepo.New("hubot", "robot-uprising"), nil
+					},
+				},
+			},
+			want:    "hubot",
+			wantErr: false,
+		},
 		{
 			name: "file",
 			args: args{
@@ -1008,6 +1022,51 @@ func Test_fillPlaceholders(t *testing.T) {
 			want:    "repos/:owner/:repo/branches/:branch",
 			wantErr: true,
 		},
+		{
+			name: "has substitutes (braces)",
+			args: args{
+				value: "repos/{owner}/{repo}/releases",
+				opts: &ApiOptions{
+					BaseRepo: func() (ghrepo.Interface, error) {
+						return ghrepo.New("hubot", "robot-uprising"), nil
+					},
+				},
+			},
+			want:    "repos/hubot/robot-uprising/releases",
+			wantErr: false,
+		},
+		{
+			name: "has branch placeholder (braces)",
+			args: args{
+				value: "repos/cli/cli/branches/{branch}/protection/required_status_checks",
+				opts: &ApiOptions{
+					BaseRepo: func() (ghrepo.Interface, error) {
+						return ghrepo.New("cli", "cli"), nil
+					},
+					Branch: func() (string, error) {
+						return "trunk", nil
+					},
+				},
+			},
+			want:    "repos/cli/cli/branches/trunk/protection/required_status_checks",
+			wantErr: false,
+		},
+		{
+			name: "has branch placeholder and git is in detached head (braces)",
+			args: args{
+				value: "repos/{owner}/{repo}/branches/{branch}",
+				opts: &ApiOptions{
+					BaseRepo: func() (ghrepo.Interface, error) {
+						return ghrepo.New("cli", "cli"), nil
+					},
+					Branch: func() (string, error) {
+						return "", git.ErrNotOnAnyBranch
+					},
+				},
+			},
+			want:    "repos/{owner}/{repo}/branches/{branch}",
+			wantErr: true,
+		},
 		{
 			name: "no greedy substitutes",
 			args: args{