cache.h       |  1 +
 environment.c |  2 +-
 sha1_name.c   | 21 ++++++++++++++++++++-
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/cache.h b/cache.h
index 6e33f2f..d2da6d1 100644
--- a/cache.h
+++ b/cache.h
@@ -1207,6 +1207,7 @@ struct object_context {
 #define GET_SHA1_TREEISH          020
 #define GET_SHA1_BLOB             040
 #define GET_SHA1_FOLLOW_SYMLINKS 0100
+#define GET_SHA1_AUTOMATIC	 0200
 #define GET_SHA1_ONLY_TO_DIE    04000
 
 #define GET_SHA1_DISAMBIGUATORS \
diff --git a/environment.c b/environment.c
index c1442df..fd6681e 100644
--- a/environment.c
+++ b/environment.c
@@ -16,7 +16,7 @@ int trust_executable_bit = 1;
 int trust_ctime = 1;
 int check_stat = 1;
 int has_symlinks = 1;
-int minimum_abbrev = 4, default_abbrev = 7;
+int minimum_abbrev = 4, default_abbrev = -1;
 int ignore_case;
 int assume_unchanged;
 int prefer_symlink_refs;
diff --git a/sha1_name.c b/sha1_name.c
index 3b647fd..8791ff3 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -15,6 +15,7 @@ typedef int (*disambiguate_hint_fn)(const unsigned char *, void *);
 
 struct disambiguate_state {
 	int len; /* length of prefix in hex chars */
+	unsigned int nrobjects;
 	char hex_pfx[GIT_SHA1_HEXSZ + 1];
 	unsigned char bin_pfx[GIT_SHA1_RAWSZ];
 
@@ -118,6 +119,12 @@ static void find_short_object_filename(struct disambiguate_state *ds)
 
 			if (strlen(de->d_name) != 38)
 				continue;
+
+			// We only look at the one subdirectory, and we assume
+			// each subdirectory is roughly similar, so each object
+			// we find probably has 255 other objects in the other
+			// fan-out directories
+			ds->nrobjects += 256;
 			if (memcmp(de->d_name, ds->hex_pfx + 2, ds->len - 2))
 				continue;
 			memcpy(hex + 2, de->d_name, 38);
@@ -151,6 +158,7 @@ static void unique_in_pack(struct packed_git *p,
 
 	open_pack_index(p);
 	num = p->num_objects;
+	ds->nrobjects += num;
 	last = num;
 	while (first < last) {
 		uint32_t mid = (first + last) / 2;
@@ -426,6 +434,12 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 		for_each_abbrev(ds.hex_pfx, show_ambiguous_object, &ds);
 	}
 
+	if (len < 16 && !status && (flags & GET_SHA1_AUTOMATIC)) {
+		unsigned int expect_collision = 1 << (len * 2);
+		if (ds.nrobjects > expect_collision)
+			return SHORT_NAME_AMBIGUOUS;
+	}
+
 	return status;
 }
 
@@ -458,14 +472,19 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
 int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
 {
 	int status, exists;
+	int flags = GET_SHA1_QUIETLY;
 
+	if (len < 0) {
+		flags |= GET_SHA1_AUTOMATIC;
+		len = 7;
+	}
 	sha1_to_hex_r(hex, sha1);
 	if (len == 40 || !len)
 		return 40;
 	exists = has_sha1_file(sha1);
 	while (len < 40) {
 		unsigned char sha1_ret[20];
-		status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
+		status = get_short_sha1(hex, len, sha1_ret, flags);
 		if (exists
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {