Add TIG_EDITOR environment variable to configure editor

Closes jonas#889

doc/tig.1.adoc

@@ -235,6 +235,11 @@ TIG_NO_DISPLAY::
 	Open Tig without rendering anything to the terminal. This force Ncurses
 	to write to /dev/null. The main use is for automated testing of Tig.
 
+TIG_EDITOR::
+	The editor command to use when visiting files. This environment
+	variable overrides $GIT_EDITOR, $EDITOR and $VISUAL, so it allows
+	to use a different editor from the one Git uses.
+
 FILES
 -----
 '$XDG_CONFIG_HOME/tig/config'::

doc/tigrc.5.adoc

@@ -67,7 +67,8 @@ the Git configuration:
 
 'core.editor'::
 
-	The editor command. Can be overridden by setting GIT_EDITOR.
+	The editor command. Can be overridden by setting TIG_EDITOR or
+	GIT_EDITOR.
 
 'core.worktree'::
 

src/display.c

@@ -132,7 +132,9 @@ open_editor(const char *file, unsigned int lineno)
 	const char *editor;
 	int argc = 0;
 
-	editor = getenv("GIT_EDITOR");
+	editor = getenv("TIG_EDITOR");
+	if (!editor)
+		editor = getenv("GIT_EDITOR");
 	if (!editor && *opt_editor)
 		editor = opt_editor;
 	if (!editor)