attr: use hashmap for attribute dictionary

The current implementation of the attribute dictionary uses a custom
hashtable.  This modernizes the dictionary by converting it to the builtin
'hashmap' structure.

Also, in order to enable a threaded API in the future add an
accompanying mutex which must be acquired prior to accessing the
dictionary of interned attributes.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

common-main.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "exec_cmd.h"
+#include "attr.h"
 
 /*
  * Many parts of Git have subprograms communicate via pipe, expect the
@@ -33,6 +34,8 @@ int main(int argc, const char **argv)
 
 	git_setup_gettext();
 
+	attr_start();
+
 	git_extract_argv0_path(argv[0]);
 
 	restore_sigpipe_to_default();