line-range: teach -L/RE/ to search relative to anchor point

Range specification -L/RE/ for blame/log unconditionally begins
searching at line one. Mailing list discussion [1] suggests that, in the
presence of multiple -L options, -L/RE/ should search relative to the
endpoint of the previous -L range, if any.

Teach the parsing machinery underlying blame's and log's -L options to
accept a start point for -L/RE/ searches. Follow-up patches will upgrade
blame and log to take advantage of this ability.

[1]: http://thread.gmane.org/gmane.comp.version-control.git/229755/focus=229966

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-range.h

@@ -9,6 +9,9 @@
  * line 'lno' inside the 'cb_data'.  The caller is expected to already
  * have a suitable map at hand to make this a constant-time lookup.
  *
+ * 'anchor' is the 1-based line at which relative range specifications
+ * should be anchored. Absolute ranges are unaffected by this value.
+ *
  * Returns 0 in case of success and -1 if there was an error.  The
  * actual range is stored in *begin and *end.  The counting starts
  * at 1!  In case of error, the caller should show usage message.
@@ -18,7 +21,7 @@ typedef const char *(*nth_line_fn_t)(void *data, long lno);
 
 extern int parse_range_arg(const char *arg,
 			   nth_line_fn_t nth_line_cb,
-			   void *cb_data, long lines,
+			   void *cb_data, long lines, long anchor,
 			   long *begin, long *end,
 			   const char *path);