line-range: teach -L^/RE/ to search from start of file

The -L/RE/ option of blame/log searches from the end of the previous -L
range, if any. Add new notation -L^/RE/ to override this behavior and
search from start of file.

The new ^/RE/ syntax is valid only as the <start> argument of
-L<start>,<end>. The <end> argument, as usual, is relative to <start>.

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

line-range.c

@@ -59,8 +59,14 @@ static const char *parse_loc(const char *spec, nth_line_fn_t nth_line,
 		return term;
 	}
 
-	if (begin < 0)
-		begin = -begin;
+	if (begin < 0) {
+		if (spec[0] != '^')
+			begin = -begin;
+		else {
+			begin = 1;
+			spec++;
+		}
+	}
 
 	if (spec[0] != '/')
 		return spec;