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.

commit-tree.c

@@ -123,7 +123,7 @@ int main(int argc, char **argv)
 	for (i = 2; i < argc; i += 2) {
 		char *a, *b;
 		a = argv[i]; b = argv[i+1];
-		if (!b || strcmp(a, "-p") || get_sha1_hex(b, parent_sha1[parents]))
+		if (!b || strcmp(a, "-p") || get_sha1(b, parent_sha1[parents]))
 			usage(commit_tree_usage);
 		check_valid(parent_sha1[parents], "commit");
 		parents++;