VCS: Add push, pull, fetch and improved diff view to VCS UI

This commit was created by merging the commits presented in #39255 for
the GSoC 2020 VCS Improvement project

VCS: Make EditorVCSInterface store less amount of internal state

VCS: Add force push checkbox + more frequent VCS updates

Add force push checkbox in the Commit dock. Also add some missing
opportunities for checking the VCS state again on from UI inputs

VCS: Fix script contents not being updated on merge conflict

VCS: Add branch creation VCS interface calls

VCS: Add VCS remote creation and remote selection menus

VCS: Show more commit information + Fix truncated commit offsets

VCS: Make VCS less noisy + Fix diff view refreshes

VCS: Fix mismatched argument names in VCS helpers

VCS: Add SSH transport support for remote operations

Also, moves the editor's VCS settings registrations to
project_settings.cpp and editor_settings.cpp

VCS: Change TTR() to vformat() for branch and remote removal text

VCS: Add VCS branch icon instead of using Tree node icon

Co-authored-by: @ChronicallySerious

core/project_settings.cpp

@@ -1061,6 +1061,9 @@ ProjectSettings::ProjectSettings() {
 	GLOBAL_DEF("editor/script_templates_search_path", "res://script_templates");
 	custom_prop_info["editor/script_templates_search_path"] = PropertyInfo(Variant::STRING, "editor/script_templates_search_path", PROPERTY_HINT_DIR);
 
+	GLOBAL_DEF("editor/version_control/autoload_on_startup", false);
+	GLOBAL_DEF("editor/version_control/plugin_name", "");
+
 	action = Dictionary();
 	action["deadzone"] = Variant(0.5f);
 	events = Array();

doc/classes/ProjectSettings.xml

@@ -517,6 +517,12 @@
 		<member name="editor/search_in_file_extensions" type="PoolStringArray" setter="" getter="" default="PoolStringArray( &quot;gd&quot;, &quot;gdshader&quot;, &quot;shader&quot; )">
 			Text-based file extensions to include in the script editor's "Find in Files" feature. You can add e.g. [code]tscn[/code] if you wish to also parse your scene files, especially if you use built-in scripts which are serialized in the scene files.
 		</member>
+		<member name="editor/version_control/autoload_on_startup" type="bool" setter="" getter="" default="false">
+			Load the previously opened VCS plugin when the editor starts up. This is set to [code]true[/code] whenever a new VCS plugin is initialized.
+		</member>
+		<member name="editor/version_control/plugin_name" type="String" setter="" getter="" default="&quot;&quot;">
+			Last loaded VCS plugin name. Used to autoload the plugin when the editor starts up.
+		</member>
 		<member name="gui/common/default_scroll_deadzone" type="int" setter="" getter="" default="0">
 			Default value for [member ScrollContainer.scroll_deadzone], which will be used for all [ScrollContainer]s unless overridden.
 		</member>

doc/classes/ScriptEditor.xml

@@ -59,6 +59,12 @@
 				Opens the script create dialog. The script will extend [code]base_name[/code]. The file extension can be omitted from [code]base_path[/code]. It will be added based on the selected scripting language.
 			</description>
 		</method>
+		<method name="reload_scripts">
+			<return type="void" />
+			<description>
+				Reload all currently opened scripts from disk in case the file contents are newer.
+			</description>
+		</method>
 	</methods>
 	<signals>
 		<signal name="editor_script_changed">

editor/editor_node.cpp

@@ -5932,6 +5932,11 @@ EditorNode::EditorNode() {
 	EDITOR_DEF("interface/inspector/default_color_picker_mode", 0);
 	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::INT, "interface/inspector/default_color_picker_mode", PROPERTY_HINT_ENUM, "RGB,HSV,RAW", PROPERTY_USAGE_DEFAULT));
 	EDITOR_DEF("run/auto_save/save_before_running", true);
+	EDITOR_DEF("version_control/username", "");
+	EDITOR_DEF("version_control/ssh_public_key_path", "");
+	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::STRING, "version_control/ssh_public_key_path", PROPERTY_HINT_GLOBAL_FILE));
+	EDITOR_DEF("version_control/ssh_private_key_path", "");
+	EditorSettings::get_singleton()->add_property_hint(PropertyInfo(Variant::STRING, "version_control/ssh_private_key_path", PROPERTY_HINT_GLOBAL_FILE));
 
 	theme_base = memnew(Control);
 	add_child(theme_base);