templater: allow trailing comma

While rewriting the default log template, I find it's annoying that I have
to remove "," from the last line.

src/commands/mod.rs

@@ -1449,7 +1449,7 @@ fn log_template(settings: &UserSettings) -> String {
                 working_copies,
                 git_head,
                 commit_id.{prefix_format},
-                if(conflict, label("conflict", "conflict"))
+                if(conflict, label("conflict", "conflict")),
               )
               "\n"
               if(empty, label("empty", "(empty)") " ")

src/template.pest

@@ -33,7 +33,7 @@ identifier = @{ (ASCII_ALPHA | "_") ~ (ASCII_ALPHANUMERIC | "_")* }
 
 function = { identifier ~ "(" ~ function_arguments ~ ")" }
 function_arguments = {
-  template ~ ("," ~ template)*
+  template ~ ("," ~ template)* ~ ("," ~ whitespace*)?
   | whitespace*
 }
 

src/template_parser.rs

@@ -789,6 +789,24 @@ mod tests {
         })
     }
 
+    #[test]
+    fn test_function_call_syntax() {
+        // Trailing comma isn't allowed for empty argument
+        assert!(parse(r#" "".first_line() "#).is_ok());
+        assert!(parse(r#" "".first_line(,) "#).is_err());
+
+        // Trailing comma is allowed for the last argument
+        assert!(parse(r#" "".contains("") "#).is_ok());
+        assert!(parse(r#" "".contains("",) "#).is_ok());
+        assert!(parse(r#" "".contains("" ,  ) "#).is_ok());
+        assert!(parse(r#" "".contains(,"") "#).is_err());
+        assert!(parse(r#" "".contains("",,) "#).is_err());
+        assert!(parse(r#" "".contains("" , , ) "#).is_err());
+        assert!(parse(r#" label("","") "#).is_ok());
+        assert!(parse(r#" label("","",) "#).is_ok());
+        assert!(parse(r#" label("",,"") "#).is_err());
+    }
+
     #[test]
     fn test_integer_literal() {
         let extract = |x: Expression<()>| x.try_into_integer().unwrap().extract(&());