revert/cherry-pick: Allow overriding the help text by the calling Porcelain

revert/cherry-pick: Allow overriding the help text by the calling Porcelain

A Porcelain command that uses cherry-pick or revert may make a commit
out of resolved index itself, in which case telling the user to commit
the result is not appropriate at all.  This allows GIT_CHERRY_PICK_HELP
environment variable to be set by the calling Porcelain in order to
override the built-in help text.

[jc: this is heavily modified from the original but should be equivalent
 in spirit]

Signed-off-by: Junio C Hamano <gitster@pobox.com>