support fetching into a shallow repository

A shallow commit is a commit which has parents, which in turn are
"grafted away", i.e. the commit appears as if it were a root.

Since these shallow commits should not be edited by the user, but
only by core git, they are recorded in the file $GIT_DIR/shallow.

A repository containing shallow commits is called shallow.

The advantage of a shallow repository is that even if the upstream
contains lots of history, your local (shallow) repository needs not
occupy much disk space.

The disadvantage is that you might miss a merge base when pulling
some remote branch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.c

@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "tag.h"
 #include "commit.h"
+#include "pkt-line.h"
 
 int save_commit_buffer = 1;
 
@@ -221,6 +222,8 @@ static void prepare_commit_graft(void)
 		return;
 	graft_file = get_graft_file();
 	read_graft_file(graft_file);
+	/* make sure shallows are read */
+	is_repository_shallow();
 	commit_graft_prepared = 1;
 }
 
@@ -234,6 +237,24 @@ static struct commit_graft *lookup_commit_graft(const unsigned char *sha1)
 	return commit_graft[pos];
 }
 
+int write_shallow_commits(int fd, int use_pack_protocol)
+{
+	int i, count = 0;
+	for (i = 0; i < commit_graft_nr; i++)
+		if (commit_graft[i]->nr_parent < 0) {
+			const char *hex =
+				sha1_to_hex(commit_graft[i]->sha1);
+			count++;
+			if (use_pack_protocol)
+				packet_write(fd, "shallow %s", hex);
+			else {
+				write(fd, hex,  40);
+				write(fd, "\n", 1);
+			}
+		}
+	return count;
+}
+
 int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size)
 {
 	char *tail = buffer;