import: don't rollback on failed import --exact (issue3616)

The checkexact() helper function was calling repo.rollback() from inside
an open transaction. In addition to being insane, this is unnecessary
because import will release the transaction on an exception.

It turns out that this has been broken since the feature was first
introduced, first released in v1.0:

changeset:   4263:47ba52121433
user:        Brendan Cully <[e-mail]>
date:        Thu Mar 22 10:44:59 2007 -0700
files:       mercurial/commands.py mercurial/patch.py
description:
Add import --exact.
When this option is set, import will apply the patch (which must
be generated by export) to the parents specified in the patch,
and check that the node produced by the patch matches the node
ID in the patch.

mercurial/commands.py

@@ -3812,7 +3812,6 b' def import_(ui, repo, patch1=None, *patc'
 
     def checkexact(repo, n, nodeid):
         if opts.get('exact') and hex(n) != nodeid:
-            repo.rollback()
             raise util.Abort(_('patch is damaged or loses information'))
 
     def tryone(ui, hunk, parents):

tests/test-import-merge.t

@@ -113,3 +113,41 b' Test with --bypass and --exact'
   $ hg strip --no-backup tip
 
   $ cd ..
+
+Test that --exact on a bad header doesn't corrupt the repo (issue3616)
+
+  $ hg init repo3
+  $ cd repo3
+  $ echo a>a
+  $ hg ci -Aqm0
+  $ echo a>>a
+  $ hg ci -m1
+  $ echo a>>a
+  $ hg ci -m2
+  $ echo a>a
+  $ echo b>>a
+  $ echo a>>a
+  $ hg ci -m3
+  $ hg export 2 | head -7 > ../a.patch
+  $ hg export tip | tail -n +8 >> ../a.patch
+
+  $ cd ..
+  $ hg clone -qr0 repo3 repo3-clone
+  $ cd repo3-clone
+  $ hg pull -qr1 ../repo3
+
+  $ hg import --exact ../a.patch
+  applying ../a.patch
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  patching file a
+  Hunk #1 succeeded at 1 with fuzz 1 (offset -1 lines).
+  transaction abort!
+  rollback completed
+  abort: patch is damaged or loses information
+  [255]
+  $ hg verify
+  checking changesets
+  checking manifests
+  crosschecking files in changesets and manifests
+  checking files
+  1 files, 2 changesets, 2 total revisions