find_unique_abbrev() simplification.

Earlier it did not grok the 0{40} SHA1 very well, but what it
needed to do was to find the shortest 0{N} that is not used as a
valid object name to be consistent with the way names of valid
objects are abbreviated.  This makes some users simpler.

Signed-off-by: Junio C Hamano <junkio@cox.net>

combine-diff.c

@@ -716,10 +716,7 @@ static int show_patch_diff(struct combine_diff_path *elem, int num_parent,
 
 	if (show_hunks || mode_differs) {
 		const char *abb;
-		char null_abb[DEFAULT_ABBREV + 1];
 
-		memset(null_abb, '0', DEFAULT_ABBREV);
-		null_abb[DEFAULT_ABBREV] = 0;
 		if (header) {
 			shown_header++;
 			puts(header);
@@ -734,17 +731,11 @@ static int show_patch_diff(struct combine_diff_path *elem, int num_parent,
 		for (i = 0; i < num_parent; i++) {
 			if (elem->parent[i].mode != elem->mode)
 				mode_differs = 1;
-			if (memcmp(elem->parent[i].sha1, null_sha1, 20))
 			abb = find_unique_abbrev(elem->parent[i].sha1,
 						 DEFAULT_ABBREV);
-			else
-				abb = null_abb;
 			printf("%s%s", i ? "," : "", abb);
 		}
-		if (memcmp(elem->sha1, null_sha1, 20))
 		abb = find_unique_abbrev(elem->sha1, DEFAULT_ABBREV);
-		else
-			abb = null_abb;
 		printf("..%s\n", abb);
 
 		if (mode_differs) {

diff.c

@@ -963,7 +963,7 @@ void diff_free_filepair(struct diff_filepair *p)
 }
 
 /* This is different from find_unique_abbrev() in that
- * it needs to deal with 0{40} SHA1.
+ * it stuffs the result with dots for alignment.
  */
 const char *diff_unique_abbrev(const unsigned char *sha1, int len)
 {
@@ -973,16 +973,8 @@ const char *diff_unique_abbrev(const unsigned char *sha1, int len)
 		return sha1_to_hex(sha1);
 
 	abbrev = find_unique_abbrev(sha1, len);
-	if (!abbrev) {
+	if (!abbrev)
-		if (!memcmp(sha1, null_sha1, 20)) {
-			char *buf = sha1_to_hex(null_sha1);
-			if (len < 37)
-				strcpy(buf + len, "...");
-			return buf;
-		}
-		else 
 		return sha1_to_hex(sha1);
-	}
 	abblen = strlen(abbrev);
 	if (abblen < 37) {
 		static char hex[41];

sha1_name.c

@@ -186,16 +186,18 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 
 const char *find_unique_abbrev(const unsigned char *sha1, int len)
 {
-	int status;
+	int status, is_null;
 	static char hex[41];
 
+	is_null = !memcmp(sha1, null_sha1, 20);
 	memcpy(hex, sha1_to_hex(sha1), 40);
 	if (len == 40)
 		return hex;
 	while (len < 40) {
 		unsigned char sha1_ret[20];
 		status = get_short_sha1(hex, len, sha1_ret, 1);
-		if (!status) {
+		if (!status ||
+		    (is_null && status != SHORT_NAME_AMBIGUOUS)) {
 			hex[len] = 0;
 			return hex;
 		}