Fix memory leak in main view

[stevenyvr987]

When the main_done() is called, done_graph() is called, which calls free(graph), where graph is a pointer to a struct graph. struct graph points to a hash table colors.id_map, which should be freed before free(graph) is called.

The memory leak can be induced in the main view by defining the keybinding on the prompt line :bind main t !t true, where true is essentially a shell no-op. Pressing t in the main view and then quitting will get a LeakSanitizer report of a direct leak. The leak happens for each t command so that the amount leaked is proportional to the number of times main_done() is called.

Direct leak of 112 byte(s) in 1 object(s) allocated from:
0x7effa34b7d38 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xded38)
0x5639427502ac in htab_create_typed_alloc compat/hashtab.c:358
0x563942750421 in htab_create_alloc compat/hashtab.c:286
0x56394274ec1c in colors_init src/graph-v2.c:219
0x56394274ec1c in get_color src/graph-v2.c:231
0x56394274ec1c in graph_generate_symbols src/graph-v2.c:727
0x56394274ec1c in graph_render_parents src/graph-v2.c:748
0x56394274533f in main_read src/main.c:461
0x5639427213bb in update_view src/view.c:611
0x56394271961b in update_views src/display.c:757
0x56394271961b in get_input src/display.c:802
0x56394270f856 in prompt_input src/prompt.c:55
0x5639426ec9ad in read_key_combo src/tig.c:745
0x5639426e9976 in main src/tig.c:855
0x7effa2967b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)