sha1_to_hex() usage cleanup

sha1_to_hex() usage cleanup

Somebody on the #git channel complained that the sha1_to_hex() thing uses
a static buffer which caused an error message to show the same hex output
twice instead of showing two different ones.

That's pretty easily rectified by making it uses a simple LRU of a few
buffers, which also allows some other users (that were aware of the buffer
re-use) to be written in a more straightforward manner.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>