commit: Restore --edit when combined with --fixup

Recent changes to --fixup, adding amend suboption, caused the
--edit flag to be ignored as use_editor was always set to zero.

Restore edit_flag having higher priority than fixup_message when
deciding the value of use_editor by only changing the default
if edit_flag is not set.

Signed-off-by: Joel Klinghed <the_jk@spawned.biz>

builtin/commit.c

@@ -1333,7 +1333,8 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		} else {
 			fixup_commit = fixup_message;
 			fixup_prefix = "fixup";
-			use_editor = 0;
+			if (0 > edit_flag)
+				use_editor = 0;
 		}
 	}