add reentrant variants of sha1_to_hex and find_unique_abbrev

The sha1_to_hex and find_unique_abbrev functions always
write into reusable static buffers. There are a few problems
with this:

  - future calls overwrite our result. This is especially
    annoying with find_unique_abbrev, which does not have a
    ring of buffers, so you cannot even printf() a result
    that has two abbreviated sha1s.

  - if you want to put the result into another buffer, we
    often strcpy, which looks suspicious when auditing for
    overflows.

This patch introduces sha1_to_hex_r and find_unique_abbrev_r,
which write into a user-provided buffer. Of course this is
just punting on the overflow-auditing, as the buffer
obviously needs to be GIT_SHA1_HEXSZ + 1 bytes. But it is
much easier to audit, since that is a well-known size.

We retain the non-reentrant forms, which just become thin
wrappers around the reentrant ones. This patch also adds a
strbuf variant of find_unique_abbrev, which will be handy in
later patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hex.c

@@ -61,12 +61,10 @@ int get_oid_hex(const char *hex, struct object_id *oid)
 	return get_sha1_hex(hex, oid->hash);
 }
 
-char *sha1_to_hex(const unsigned char *sha1)
+char *sha1_to_hex_r(char *buffer, const unsigned char *sha1)
 {
-	static int bufno;
-	static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];
 	static const char hex[] = "0123456789abcdef";
-	char *buffer = hexbuffer[3 & ++bufno], *buf = buffer;
+	char *buf = buffer;
 	int i;
 
 	for (i = 0; i < GIT_SHA1_RAWSZ; i++) {
@@ -79,6 +77,13 @@ char *sha1_to_hex(const unsigned char *sha1)
 	return buffer;
 }
 
+char *sha1_to_hex(const unsigned char *sha1)
+{
+	static int bufno;
+	static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];
+	return sha1_to_hex_r(hexbuffer[3 & ++bufno], sha1);
+}
+
 char *oid_to_hex(const struct object_id *oid)
 {
 	return sha1_to_hex(oid->hash);