repair: open a file with checkambig=True to avoid file stat ambiguity

Before this patch, if steps below occurs at "the same time in sec",
all of mtime, ctime and size are same between (1) and (3).

  1. append data to revlog-style file (and close transaction)
  2. discard appended data by truncation of strip
  3. append same size but different data to revlog-style file again

Therefore, cache validation doesn't work after (3) as expected.

To avoid such file stat ambiguity around truncation, this patch opens
a file with checkambig=True.

This patch also introduces "with" statement style, to ensure immediate
invocation of close() after truncation, because closing file is the
only trigger to check (and get rid of) file stat ambiguity.

This is a part of ExactCacheValidationPlan.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

mercurial/repair.py

@@ -181,7 +181,8 b' def strip(ui, repo, nodelist, backup=Tru'
 
             for i in xrange(offset, len(tr.entries)):
                 file, troffset, ignore = tr.entries[i]
-                repo.svfs(file, 'a').truncate(troffset)
+                with repo.svfs(file, 'a', checkambig=True) as fp:
+                    fp.truncate(troffset)
                 if troffset == 0:
                     repo.store.markremoved(file)