git-pickaxe: allow "-L <something>,+N"

With this,

	git pickaxe -L '/--progress/,+20' v1.4.0 -- pack-objects.c

gives you 20 lines starting from the first occurrence of
'--progress' in pack-objects, digging from v1.4.0 version.

You can also say

	git pickaxe -L '/--progress/,-5' v1.4.0 -- pack-objects.c
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-pickaxe.c

@@ -1545,6 +1545,25 @@ static const char *parse_loc(const char *spec,
 	regex_t regexp;
 	regmatch_t match[1];
 
+	/* Allow "-L <something>,+20" to mean starting at <something>
+	 * for 20 lines, or "-L <something>,-5" for 5 lines ending at
+	 * <something>.
+	 */
+	if (1 < begin && (spec[0] == '+' || spec[0] == '-')) {
+		num = strtol(spec + 1, &term, 10);
+		if (term != spec + 1) {
+			if (spec[0] == '-')
+				num = 0 - num;
+			if (0 < num)
+				*ret = begin + num - 2;
+			else if (!num)
+				*ret = begin;
+			else
+				*ret = begin + num;
+			return term;
+		}
+		return spec;
+	}
 	num = strtol(spec, &term, 10);
 	if (term != spec) {
 		*ret = num;