diff: change id abbrev option's name to id_abbrev

Same as the other commits in the series, we use 'id' when talking about
thing rather than the datatype.

include/git2/diff.h

@@ -361,7 +361,7 @@ typedef struct {
 
 	uint16_t    context_lines;    /**< defaults to 3 */
 	uint16_t    interhunk_lines;  /**< defaults to 0 */
-	uint16_t    oid_abbrev;       /**< default 'core.abbrev' or 7 if unset */
+	uint16_t    id_abbrev;       /**< default 'core.abbrev' or 7 if unset */
 	git_off_t   max_size;         /**< defaults to 512MB */
 	const char *old_prefix;       /**< defaults to "a" */
 	const char *new_prefix;       /**< defaults to "b" */

src/diff_print.c

@@ -37,8 +37,8 @@ static int diff_print_info_init(
 	if (diff)
 		pi->flags = diff->opts.flags;
 
-	if (diff && diff->opts.oid_abbrev != 0)
-		pi->oid_strlen = diff->opts.oid_abbrev;
+	if (diff && diff->opts.id_abbrev != 0)
+		pi->oid_strlen = diff->opts.id_abbrev;
 	else if (!diff || !diff->repo)
 		pi->oid_strlen = GIT_ABBREV_DEFAULT;
 	else if (git_repository__cvar(