Remove pack file handling dependency from wrapper.o

As v1.7.0-rc0~43 (slim down "git show-index", 2010-01-21) explains,
use of xmalloc() brings in a dependency on zlib, the sha1 lib, and the
rest of git's object file access machinery via try_to_free_pack_memory.
That is overkill when xmalloc is just being used as a convenience
wrapper to exit when no memory is available.

So defer setting try_to_free_pack_memory as try_to_free_routine until
the first packfile is opened in add_packed_git().

After this change, a simple program using xmalloc() and no other
functions will not pull in any code from libgit.a aside from wrapper.o
and usage.o.
Improved-by: NRené Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -818,11 +818,22 @@ static struct packed_git *alloc_packed_git(int extra)
 	return p;
 }
 
+static void try_to_free_pack_memory(size_t size)
+{
+	release_pack_memory(size, -1);
+}
+
 struct packed_git *add_packed_git(const char *path, int path_len, int local)
 {
+	static int have_set_try_to_free_routine;
 	struct stat st;
 	struct packed_git *p = alloc_packed_git(path_len + 2);
 
+	if (!have_set_try_to_free_routine) {
+		have_set_try_to_free_routine = 1;
+		set_try_to_free_routine(try_to_free_pack_memory);
+	}
+
 	/*
 	 * Make sure a corresponding .pack file exists and that
 	 * the index looks sane.

wrapper.c

@@ -3,12 +3,11 @@
  */
 #include "cache.h"
 
-static void try_to_free_builtin(size_t size)
+static void do_nothing(size_t size)
 {
-	release_pack_memory(size, -1);
 }
 
-static void (*try_to_free_routine)(size_t size) = try_to_free_builtin;
+static void (*try_to_free_routine)(size_t size) = do_nothing;
 
 try_to_free_t set_try_to_free_routine(try_to_free_t routine)
 {