cgit_print_tree(): Free curr_rev after usage

cgit_print_tree(): Free curr_rev after usage

Fixes following memory leak seen with "PATH_INFO=/cgit/tree/":

    ==15715== 7 bytes in 1 blocks are definitely lost in loss record 4 of 51
    ==15715==    at 0x4C2C04B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==15715==    by 0x56F2DF1: strdup (in /usr/lib/libc-2.17.so)
    ==15715==    by 0x46CAA8: xstrdup (wrapper.c:35)
    ==15715==    by 0x418A4C: cgit_print_tree (ui-tree.c:274)
    ==15715==    by 0x407D91: tree_fn (cmd.c:131)
    ==15715==    by 0x405E16: process_request (cgit.c:574)
    ==15715==    by 0x4074C8: cache_process (cache.c:322)
    ==15715==    by 0x406C4F: main (cgit.c:872)

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>