sha1_name: make core.warnambiguousrefs the default.

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

environment.c

@@ -14,7 +14,7 @@ char git_default_name[MAX_GITNAME];
 int trust_executable_bit = 1;
 int assume_unchanged = 0;
 int only_use_symrefs = 0;
-int warn_ambiguous_refs = 0;
+int warn_ambiguous_refs = 1;
 int repository_format_version = 0;
 char git_commit_encoding[MAX_ENCODING_LENGTH] = "utf-8";
 int shared_repository = 0;