graft: moved abortgraft and readgraft to cmdutil
This patch moves `abortgraft` and `readgraft` to
`cmdutil`. Various callers are updated accordingly.
This is done because these serve as ulitlity functions
for command `graft` and so that new functions regarding
graft can be built from them.
Differential Revision: https://phab.mercurial-scm.org/D6608