Allow applying a patch with conflicts

Allow applying a patch with conflicts

In some settings, we want to let users apply a patch that does
not cleanly apply and add conflict markers. In Gerrit, this is
useful when cherry picking (via Git patches) from one host to
another.

This commit takes a simple approach: If a hunk doesn't apply,
go to the pre-image line, treat all lines in pre-image length
as left side of the conflict and all context and newly added
lines as right side of the conflict.

Change-Id: I01411d7a32b3f3207097b26231909aae6b835650