Add documentation for new git history limiting configuration (#764)

Author: vim-zz

docs/automation-actions.md

@@ -500,6 +500,10 @@ automations:
 
     For more information about the `codeExperts` filter function, see the [filter functions documentation](https://docs.gitstream.cm/filter-functions/#codeexperts).
 
+!!! tip "Limit git history for code experts"
+    
+    Use the [`config.git_history_since`](./cm-file.md#configgit_history_since) configuration to limit the git history analysis to commits after a specific date. This is useful for team transitions or when you want to focus on recent contributors only.
+
 #### `merge` :fontawesome-brands-github: :fontawesome-brands-gitlab: :fontawesome-brands-bitbucket:
 
 Once triggered, merge the PR if possible. It can be set to wait for all checks to pass or only required ones.

docs/cm-file.md

@@ -171,12 +171,13 @@ The manifest version field is used to parse the `.cm` file, in the future if bre
 
 The `config` section in the `.cm` file is optional and specifies settings that affect gitStream's operation within a given context.
 
-| Key                   | Type             | Default | Scope          | Description                                                                         |
-| --------------------- | ---------------- | ------- | -------------- | ----------------------------------------------------------------------------------- |
-| `config`              | Map              | -       | per `.cm` file | Root configuration section, applies to the automations defined in the current file. |
-| `config.admin.users`  | [String]         | `[]`    | `gitstream.cm` | List of admin users, identified by Git provider usernames.                          |
-| `config.ignore_files` | [String]         | `[]`    | per `.cm` file | Files to exclude from consideration.                                                |
-| `config.user_mapping` | [String: String] | `[]`    | per `.cm` file | Map Git user details to provider account names.                                     |
+| Key                      | Type             | Default | Scope          | Description                                                                         |
+| ------------------------ | ---------------- | ------- | -------------- | ----------------------------------------------------------------------------------- |
+| `config`                 | Map              | -       | per `.cm` file | Root configuration section, applies to the automations defined in the current file. |
+| `config.admin.users`     | [String]         | `[]`    | `gitstream.cm` | List of admin users, identified by Git provider usernames.                          |
+| `config.git_history_since` | String         | -       | per `.cm` file | Limit git history analysis to commits after the specified date (YYYY-MM-DD).       |
+| `config.ignore_files`    | [String]         | `[]`    | per `.cm` file | Files to exclude from consideration.                                                |
+| `config.user_mapping`    | [String: String] | `[]`    | per `.cm` file | Map Git user details to provider account names.                                     |
 
 ##### `config.admin.users`
 
@@ -247,6 +248,46 @@ On the other hand, when using `explainRankByGitBlame` with `add-comment@v1` it s
 1.  `rankByGitBlame` will drop `null` users
 2.  `explainRankByGitBlame` will NOT drop `null` users
 
+##### `config.git_history_since`
+
+The `config.git_history_since` configuration limits how far back gitStream looks in git history when analyzing code expertise and git blame results. This affects filters and actions that rely on git history analysis, including:
+
+- `codeExperts` filter
+- `rankByGitBlame` filter  
+- `explainRankByGitBlame` filter
+- `explain-code-experts` action
+
+When `git_history_since` is configured, gitStream will only consider commits made on or after the specified date when determining code experts and calculating git blame rankings. This is particularly useful for:
+
+- **Team transitions**: When a project changes hands between teams, you can set the date to when the new team took over to ensure only current team members are suggested as reviewers
+- **Performance optimization**: Large repositories with extensive history can benefit from limiting the analysis scope
+- **Relevant expertise**: Focus on recent contributions rather than legacy code changes
+
+The value should be specified as a date in `YYYY-MM-DD` format.
+
+```yaml title="example"
+config:
+  git_history_since: '2025-01-01'  # Only consider commits from January 1, 2025 onwards
+```
+
+**Example use case**: If your team took over a project on January 1, 2025, and you want to ensure that only current team members are suggested as code experts:
+
+```yaml title=".cm/gitstream.cm"
+config:
+  git_history_since: '2025-01-01'
+
+automations:
+  assign_code_experts:
+    if:
+      - {{ files | codeExperts(gt=10) | length > 0 }}
+    run:
+      - action: add-reviewers@v1
+        args:
+          reviewers: {{ files | codeExperts(gt=10) }}
+```
+
+With this configuration, the `codeExperts` filter will only analyze git blame data from commits made on or after January 1, 2025, effectively filtering out previous team members from reviewer suggestions.
+
 #### `triggers`
 
 The `triggers` is section specifies when automations are executed, supporting `include` and `exclude` lists for branch and repository patterns at the file level. The `on` keyword can also be used within individual automations to define specific events that trigger those automations.

docs/filter-functions.md

@@ -415,6 +415,10 @@ automations:
           reviewers: {{ repo | codeExperts(gt=10) }}
 ```
 
+!!! tip "Limit git history for code experts"
+    
+    Use the [`config.git_history_since`](./cm-file.md#configgit_history_since) configuration to limit the git history analysis to commits after a specific date. This is useful for team transitions or when you want to focus on recent contributors only.
+
 #### `decode`
 
 Decode Base64 encoded string into an object. Encoded strings are formatted: `"base64: <encoded_string>"`