Add "get_sha1()" helper function.

This allows the programs to use various simplified versions of
the SHA1 names, eg just say "HEAD" for the SHA1 pointed to by
the .git/HEAD file etc.

For example, this commit has been done with

	git-commit-tree $(git-write-tree) -p HEAD

instead of the traditional "$(cat .git/HEAD)" syntax.

diff-cache.c

@@ -175,7 +175,7 @@ int main(int argc, char **argv)
 		usage(diff_cache_usage);
 	}
 
-	if (argc != 2 || get_sha1_hex(argv[1], tree_sha1))
+	if (argc != 2 || get_sha1(argv[1], tree_sha1))
 		usage(diff_cache_usage);
 
 	mark_merge_entries();