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.

cat-file.c

@@ -12,7 +12,7 @@ int main(int argc, char **argv)
 	void *buf;
 	unsigned long size;
 
-	if (argc != 3 || get_sha1_hex(argv[2], sha1))
+	if (argc != 3 || get_sha1(argv[2], sha1))
 		usage("cat-file [-t | tagname] <sha1>");
 	buf = read_sha1_file(sha1, type, &size);
 	if (!buf)