graft: start storing new nodes formed in graftstate This patch starts storing the new nodes formed during the ongoing graft operation in the graftstate. We need the list of new nodes formed while implmenting `graft --abort` which will strip out the new nodes. Differential Revision: https://phab.mercurial-scm.org/D3751

show: use filter() function to strip "tip" tag Before, an empty tag "" was inserted in place of "tip", resulting in double spaces.

templater: extend filter() to accept template expression for emptiness test This utilizes the pass-by-name nature of template arguments.