Correct optimization from 3464f5e77f34; add a test.
If there are two (or more) heads that point to the same .hgtags
node, we can safely skip parsing the file in all but the last head.
(In 3464f5e77f34, we were parsing the file in the first head and
skipping all the others.)
EVERYTHING
diff -r ec612a6291f1 alpha/one
--- a/alpha/one
+++ b/alpha/one
@@ -0,0 +1,1 @@
+1
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2
BETA ONLY
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2
INSIDE BETA
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2