Increase core.packedGitLimit

When core.packedGitLimit is exceeded, git will close packs.  If there
is a repack operation going on in parallel with a fetch, the fetch
might open a pack, and then be forced to close it due to
packedGitLimit being hit.  The repack could then delete the pack
out from under the fetch, causing the fetch to fail.

Increase core.packedGitLimit's default value to prevent
this.

On current 64-bit x86_64 machines, 48 bits of address space are
available.  It appears that 64-bit ARM machines have no standard
amount of address space (that is, it varies by manufacturer), and IA64
and POWER machines have the full 64 bits.  So 48 bits is the only
limit that we can reasonably care about.  We reserve a few bits of the
48-bit address space for the kernel's use (this is not strictly
necessary, but it's better to be safe), and use up to the remaining
45.  No git repository will be anywhere near this large any time soon,
so this should prevent the failure.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -616,7 +616,7 @@ extern int git_lstat(const char *, struct stat *);
 #endif
 
 #define DEFAULT_PACKED_GIT_LIMIT \
-	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? 8192 : 256))
+	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? (32 * 1024L * 1024L) : 256))
 
 #ifdef NO_PREAD
 #define pread git_pread