more completely solve catKey memory leak

Done using a mode witness, which ensures it's fixed everywhere.

Fixing catFileKey was a bear, because git cat-file does not provide a
nice way to query for the mode of a file and there is no other efficient
way to do it. Oh, for libgit2..

Note that I am looking at tree objects from HEAD, rather than the index.
Because I cat-file cannot show a tree object for the index.
So this fix is technically incomplete. The only cases where it matters
are:

1. A new large file has been directly staged in git, but not committed.
2. A file that was committed to HEAD as a symlink has been staged
   directly in the index.

This could be fixed a lot better using libgit2.

doc/bugs/Huge_annex_out_of_memory_on_switch_to_indirect_mode_and_status.mdwn

@@ -61,3 +61,9 @@ commit  git-annex: out of memory (requested 985661440 bytes)
 
 # End of transcript or log.
 """]]
+
+> [[fixed|done]]. However, if you saw this behavior,
+> you have large files checked directly into git. You may
+> want to examine your repository and use git filter-branch to clean
+> it up.
+> --[[Joey]] 

doc/bugs/Out_of_memory_error_in_fsck_whereis_find_and_status_cmds.mdwn

@@ -77,4 +77,8 @@ Any thoughts on how I can get git-annex (esp. fsck) to complete would be appreci
 Thanks
 Giovanni
 
-[[!tag moreinfo]]
+> [[fixed|done]]. However, if you saw this behavior,
+> you have large files checked directly into git. You may
+> want to examine your repository and use git filter-branch to clean
+> it up.
+> --[[Joey]]