Use correct line when using :parent in blame view

This fixes the bug described in the parent commit.

`blame->lineno` is only used here and in #1370, so
we can either do this or we could shift all the
entries in `blame->lineno` by 1.

Author: ilyagr

src/blame.c

@@ -335,7 +335,7 @@ setup_blame_parent_line(struct view *view, struct blame *blame)
 				blamed_lineno = atoi(pos + 1);
 
 		} else if (*line == '+' && parent_lineno != -1) {
-			if (blame->lineno == blamed_lineno - 1 &&
+			if (blame->lineno == blamed_lineno &&
 			    !strcmp(blame->text, line + 1)) {
 				view->pos.lineno = parent_lineno ? parent_lineno - 1 : 0;
 				break;

test/blame/navigation-parent-test

@@ -22,8 +22,9 @@ steps '
 
 test_tig blame +36 project/Build.scala
 
-# This demonstrates a BUG: after pressing ',' we stay at line 36, as shown in the status
-# bar. Line 36 has nothing to do with line 36 of the original commit.
+# This shows that after pressing ',' we should be positioned on line 40
+# (starting with "clean in common"), as shown in the status bar. We should *not*
+# stay on line 36.
 assert_equals 'recursive-blame.screen' <<EOF
 90286e0 Jonas Fonseca 2013-10-14 14:56 -0400  22|           "-feature",
 90286e0 Jonas Fonseca 2013-10-14 14:56 -0400  23|           "-encoding", "utf8"
@@ -53,5 +54,5 @@ assert_equals 'recursive-blame.screen' <<EOF
 4edd069 Jonas Fonseca 2013-10-17 20:34 -0400  47|       richards,
 90286e0 Jonas Fonseca 2013-10-14 14:56 -0400  48|       tracer
 90286e0 Jonas Fonseca 2013-10-14 14:56 -0400  49|   )
-[blame] 90286e0752016a6bca30dfa7ca236d1f99345eb8 changed project/Build.scala - line 36 of 63                                                                                                         77%
+[blame] 90286e0752016a6bca30dfa7ca236d1f99345eb8 changed project/Build.scala - line 40 of 63                                                                                                         77%
 EOF