Merge branch 'rs/fix-alt-odb-path-comparison' into maint

Code to avoid adding the same alternate object store twice was
subtly broken for a long time, but nobody seems to have noticed.

* rs/fix-alt-odb-path-comparison:
  sha1_file: avoid overrunning alternate object base string

Author: gitster

sha1_file.c

@@ -315,7 +315,8 @@ static int link_alt_odb_entry(const char *entry, const char *relative_base, int
 	 * thing twice, or object directory itself.
 	 */
 	for (alt = alt_odb_list; alt; alt = alt->next) {
-		if (!memcmp(ent->base, alt->base, pfxlen)) {
+		if (pfxlen == alt->name - alt->base - 1 &&
+		    !memcmp(ent->base, alt->base, pfxlen)) {
 			free(ent);
 			return -1;
 		}