test-convert-git: stabilize corruption of test git repo

This error would show up only intermittently since the
test depended on the order of the directories returned by os.walk.

The damage repository test would delete the first object file it came
across. However, the order of the directory listing is arbitrary (it
seems to depend on the filesystem). This meant that sometimes a commit
object was deleted, sometimes a blob object and sometimes a tree
object.

So, fix by hardcoding which object to delete. Delete a commit object,
a blob object and a tree object in three separate tests.

tests/test-convert-git.t

@@ -281,24 +281,6 b' convert author committer'
   abort: --sourcesort is not supported by this data source
   [255]
 
-damage git repository and convert again
-
-  $ cat > damage.py <<EOF
-  > import os
-  > import stat
-  > for root, dirs, files in os.walk('git-repo4/.git/objects'):
-  >     if files:
-  >         path = os.path.join(root, files[0])
-  >         if os.name == 'nt':
-  >             os.chmod(path, stat.S_IWUSR)
-  >         os.remove(path)
-  >         break
-  > EOF
-  $ python damage.py
-  $ hg convert git-repo4 git-repo4-broken-hg 2>&1 | \
-  >     grep 'abort:' | sed 's/abort:.*/abort:/g'
-  abort:
-
 test sub modules
 
   $ mkdir git-repo5
@@ -345,3 +327,32 b' convert sub modules'
   $ cd git-repo5
   $ cat foo
   sub
+
+  $ cd ../..
+
+damaged git repository tests:
+In case the hard-coded hashes change, the following commands can be used to
+list the hashes and their corresponding types in the repository:
+cd git-repo4/.git/objects
+find . -type f | cut -c 3- | sed 's_/__' | xargs -n 1 -t git cat-file -t
+cd ../../..
+
+damage git repository by renaming a commit object
+  $ COMMIT_OBJ=1c/0ce3c5886f83a1d78a7b517cdff5cf9ca17bdd
+  $ mv git-repo4/.git/objects/$COMMIT_OBJ git-repo4/.git/objects/$COMMIT_OBJ.tmp
+  $ hg convert git-repo4 git-repo4-broken-hg 2>&1 | grep 'abort:'
+  abort: cannot read tags from git-repo4/.git
+  $ mv git-repo4/.git/objects/$COMMIT_OBJ.tmp git-repo4/.git/objects/$COMMIT_OBJ
+damage git repository by renaming a blob object
+
+  $ BLOB_OBJ=8b/137891791fe96927ad78e64b0aad7bded08bdc
+  $ mv git-repo4/.git/objects/$BLOB_OBJ git-repo4/.git/objects/$BLOB_OBJ.tmp
+  $ hg convert git-repo4 git-repo4-broken-hg 2>&1 | grep 'abort:'
+  abort: cannot read 'blob' object at 8b137891791fe96927ad78e64b0aad7bded08bdc
+  $ mv git-repo4/.git/objects/$BLOB_OBJ.tmp git-repo4/.git/objects/$BLOB_OBJ
+damage git repository by renaming a tree object
+
+  $ TREE_OBJ=72/49f083d2a63a41cc737764a86981eb5f3e4635
+  $ mv git-repo4/.git/objects/$TREE_OBJ git-repo4/.git/objects/$TREE_OBJ.tmp
+  $ hg convert git-repo4 git-repo4-broken-hg 2>&1 | grep 'abort:'
+  abort: cannot read changes in 1c0ce3c5886f83a1d78a7b517cdff5cf9ca17bdd