git-p4: simplify regex pattern generation for parsing diff-tree

git-p4: simplify regex pattern generation for parsing diff-tree

It is not clear why a generator was used to create the regex used to
parse git-diff-tree output; I assume an early implementation required
it, but is not part of the mainline change.

Simply use a lazily initialized global instead.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Reviewed-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>