[PATCH] Kill a bunch of pointer sign warnings for gcc4

- Raw hashes should be unsigned char.
- String functions want signed char.
- Hash and compress functions want unsigned char.

Signed-off By: Brian Gerst [off-list ref]
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PS.
tar-tree.c: In function ‘main’:
tar-tree.c:437: warning: pointer targets in passing argument 1 of 
‘write_header’ differ in signedness

The "0" looks bogus, since it should be a raw hash not a text string.