Fix RawParseUtils.formatBase10 to work with negative values

Change-Id: Iffa220de76c5e180796fa46c4d67f52a1b3b2e35
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/util/RawParseUtils.java

@@ -155,6 +155,8 @@ public final class RawParseUtils {
 			return o;
 		}
 		final boolean isneg = value < 0;
+		if (isneg)
+			value = -value;
 		while (value != 0) {
 			b[--o] = base10byte[value % 10];
 			value /= 10;