Avoid wasting memory in git-rev-list

As pointed out on the list, git-rev-list can use a lot of memory.

One low-hanging fruit is to free the commit buffer for commits that we
parse. By default, parse_commit() will save away the buffer, since a lot
of cases do want it, and re-reading it continually would be unnecessary. 
However, in many cases the buffer isn't actually necessary and saving it 
just wastes memory.

We could just free the buffer ourselves, but especially in git-rev-list, 
we actually end up using the helper functions that automatically add 
parent commits to the commit lists, so we don't actually control the 
commit parsing directly. 

Instead, just make this behaviour of "parse_commit()" a global flag.  
Maybe this is a bit tasteless, but it's very simple, and it makes a
noticable difference in memory usage.

Before the change:

	[torvalds@g5 linux]$ /usr/bin/time git-rev-list v2.6.12..HEAD > /dev/null
	0.26user 0.02system 0:00.28elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+3714minor)pagefaults 0swaps

after the change:

	[torvalds@g5 linux]$ /usr/bin/time git-rev-list v2.6.12..HEAD > /dev/null
	0.26user 0.00system 0:00.27elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
	0inputs+0outputs (0major+2433minor)pagefaults 0swaps

note how the minor faults have decreased from 3714 pages to 2433 pages. 
That's all due to the fewer anonymous pages allocated to hold the comment 
buffers and their metadata.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

Hey, I'm not proud. If somebody wants to do a prettier patch, go wild.

This still doesn't attack the _big_ memory use by

	"git-rev-list --objects"

where we just parse more stuff than we need to, but it's a first step.

diff --git a/commit.c b/commit.c
--- a/commit.c
+++ b/commit.c

@@ -3,6 +3,8 @@
 #include "commit.h"
 #include "cache.h"
 
+int save_commit_buffer = 1;
+
 struct sort_node
 {
 	/*

@@ -264,7 +266,7 @@ int parse_commit(struct commit *item)
 			     sha1_to_hex(item->object.sha1));
 	}
 	ret = parse_commit_buffer(item, buffer, size);
-	if (!ret) {
+	if (save_commit_buffer && !ret) {
 		item->buffer = buffer;
 		return 0;
 	}

diff --git a/commit.h b/commit.h
--- a/commit.h
+++ b/commit.h

@@ -17,6 +17,7 @@ struct commit {
 	char *buffer;
 };
 
+extern int save_commit_buffer;
 extern const char *commit_type;
 
 struct commit *lookup_commit(const unsigned char *sha1);

diff --git a/rev-list.c b/rev-list.c
--- a/rev-list.c
+++ b/rev-list.c

@@ -582,6 +582,8 @@ int main(int argc, char **argv)
 		handle_one_commit(commit, &list);
 	}
 
+	save_commit_buffer = verbose_header;
+
 	if (!merge_order) {		
 		sort_by_date(&list);
 	        if (limited)