github: use OAuth2/web flow for GitHub

Add an OAuth2 web browser-based authentication flow to replace the
username & password flow for GitHub.com and augment the u/p flow for
GHE instances.

Author: mjcheetham

docs/configuration.md

@@ -136,8 +136,33 @@ Configure GCM Core to use the a proxy for network operations.
 
 **Note:** Git itself does _not_ respect this setting; this affects GCM _only_.
 
+#### Example
+
 ```shell
 git config --global credential.httpsProxy http://john.doe:[email protected]
 ```
 
 **Also see: [GCM_HTTP_PROXY](environment.md#GCM_HTTP_PROXY-deprecated)**
+
+---
+
+### credential.gitHubAuthModes
+
+Override the available authentication modes presented during GitHub authentication.
+If this option is not set, then the available authentication modes will be automatically detected.
+
+**Note:** This setting supports multiple values separated by spaces.
+
+Value|Authentication Mode
+-|-
+_(unset)_|Automatically detect modes
+`oauth`|OAuth-based authentication
+`basic`|Basic/PAT-based authentication
+
+#### Example
+
+```shell
+git config --global credential.gitHubAuthModes "oauth basic"
+```
+
+**Also see: [GCM_GITHUB_AUTHMODES](environment.md#GCM_GITHUB_AUTHMODES)**

docs/environment.md

@@ -280,3 +280,32 @@ export GCM_HTTP_PROXY=http://john.doe:[email protected]
 ```
 
 **Also see: [credential.httpProxy](configuration.md#credentialhttpProxy-deprecated)**
+
+---
+
+### GCM_GITHUB_AUTHMODES
+
+Override the available authentication modes presented during GitHub authentication.
+If this option is not set, then the available authentication modes will be automatically detected.
+
+**Note:** This setting supports multiple values separated by spaces.
+
+Value|Authentication Mode
+-|-
+_(unset)_|Automatically detect modes
+`oauth`|OAuth-based authentication
+`basic`|Basic/PAT-based authentication
+
+##### Windows
+
+```batch
+SET GCM_GITHUB_AUTHMODES="oauth basic"
+```
+
+##### macOS/Linux
+
+```bash
+export GCM_GITHUB_AUTHMODES="oauth basic"
+```
+
+**Also see: [credential.gitHubAuthModes](configuration.md#credentialgitHubAuthModes)**