feat: Add the hg_branch module (#569)

docs/config/README.md

@@ -93,6 +93,7 @@ prompt_order = [
     "git_branch",
     "git_state",
     "git_status",
+    "hg_branch",
     "package",
     "dotnet",
     "golang",
@@ -559,6 +560,31 @@ The module will be shown if any of the following conditions are met:
 symbol = "🏎💨 "
 ```
 
+## Mercurial Branch
+
+The `hg_branch` module shows the active branch of the repo in your current directory.
+
+### Options
+
+| Variable            | Default         | Description                                                                                  |
+| ------------------- | --------------- | -------------------------------------------------------------------------------------------- |
+| `symbol`            | `" "`          | The symbol used before the hg bookmark or branch name of the repo in your current directory. |
+| `truncation_length` | `2^63 - 1`      | Truncates the hg branch name to X graphemes                                                  |
+| `truncation_symbol` | `"…"`           | The symbol used to indicate a branch name was truncated.                                     |
+| `style`             | `"bold purple"` | The style for the module.                                                                    |
+| `disabled`          | `true`          | Disables the `hg_branch` module.                                                             |
+
+### Example
+
+```toml
+# ~/.config/starship.toml
+
+[hg_branch]
+symbol = "🌱 "
+truncation_length = 4
+truncation_symbol = ""
+```
+
 ## Hostname
 
 The `hostname` module shows the system hostname.