git-convert-cache: fix up mode conversion

The old mode conversion was not only complex, it also refused to change
the length of a mode, which made it fragile.  By moving the mode
conversion around a bit, we can not only simplify it, it also ends up
being more powerful.

Also fix a memory leak that made it impossible to convert huge archives
without tons and tons of memory.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>