git p4: refactor diffOpts calculation

git p4: refactor diffOpts calculation

P4Submit.applyCommit()

To avoid recalculating the same diffOpts for each commit, move it
out of applyCommit() and into the top-level run().  Also fix a bug
in that code which interpreted the value of detectRenames as a
string rather than as a boolean.

[pw: fix documentation, rearrange code a bit]

Signed-off-by: Gary Gibbons <ggibbons@perforce.com>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>