diff_aligned_abbrev: use "struct oid"

Since we're modifying this function anyway, it's a good time
to update it to the more modern "struct oid". We can also
drop some of the magic numbers in favor of GIT_SHA1_HEXSZ,
along with some descriptive comments.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index f2b04b6..01afc70 100644
--- a/diff.h
+++ b/diff.h
@@ -344,7 +344,7 @@ extern void diff_warn_rename_limit(const char *varname, int needed, int degraded
  * This is different from find_unique_abbrev() in that
  * it stuffs the result with dots for alignment.
  */
-extern const char *diff_aligned_abbrev(const unsigned char *sha1, int);
+extern const char *diff_aligned_abbrev(const struct object_id *sha1, int);
 
 /* do not report anything on removed paths */
 #define DIFF_SILENT_ON_REMOVED 01