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.

fsck-cache.c

@@ -178,7 +178,7 @@ int main(int argc, char **argv)
 		if (*arg == '-')
 			continue;
 
-		if (!get_sha1_hex(arg, head_sha1)) {
+		if (!get_sha1(arg, head_sha1)) {
 			struct commit *commit = lookup_commit(head_sha1);
 			struct object *obj;