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merge3: correctly handle 1-based closed intervals

the previous bug wasn't a missing clamp, but a
mishandling of the 1-based closed intervals that
we were genrating internally, and some asserts
that assumed open intervals.

--- a/sys/src/cmd/diff/merge3.c

+++ b/sys/src/cmd/diff/merge3.c

@@ -16,8 +16,6 @@

 	if (a > b && c > d)

 		return;

-	if(c > d)

-		d = c;

 	if(df->nchanges%1024 == 0)

 		df->changes = erealloc(df->changes, (df->nchanges+1024)*sizeof(df->changes[0]));

 	ch = &df->changes[df->nchanges++];

@@ -72,8 +70,7 @@

 	ry = rc->newy;

 	if(ly - lx != ry - rx)

 		return 0;

-	assert(lx <= ly && ly <= l->len[1]);

-	assert(rx <= ry && ry <= r->len[1]);

+	assert(ly <= l->len[1] && ry <= r->len[1]);

 	Bseek(l->input[1], l->ixnew[lx-1], 0);

 	Bseek(r->input[1], r->ixnew[rx-1], 0);

 	for(i = 0; i <= (ly - lx); i++){

@@ -159,7 +156,7 @@

 				Bprint(&stdout, ">>>>>>>>>>\n");

 				status = "conflict";

 			}

-			ln = y+1;

+			ln = (y > x) ? y+1 : x+1;

 			il++;

 			ir++;

 		}else if(lc != nil && (rc == nil || lx < rx)){

@@ -175,7 +172,7 @@

 		}else

 			abort();

 	}

-	if(ln < l->len[0])

+	if(ln <= l->len[0])

 		fetch(l, l->ixold, ln, l->len[0], l->input[0], "");

 	return status;

 }

-- 

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