add prechangegroup and pretxnchangegroup hooks.
prechangegroup lets you stop push, pull or unbundle before it begins.
pretxnchangegroup lets you inspect changegroup before transaction is
committed, and roll back if you not like it.
1:f4d7a8c73d23
0:232e179b3f29
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232e179b3f29
232e179b3f29+
232e179b3f29
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merge: warning: conflicts during merge
merging file1
merging file1 failed!
diff -r f4d7a8c73d23 file1
--- a/file1
+++ b/file1
@@ -1,3 +1,7 @@ added file1
added file1
another line of text
+<<<<<<<
+changed file1 different
+=======
changed file1
+>>>>>>>
M file1
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