Permit cache.diff.intraline to disable MyersDiff

I can't fix the infinite loop in JGit's MyersDiff implementation in
a reasonable time frame.  But its a data dependent bug that triggers
only rarely, most of the time the intraline difference feature works
"well enough" to be useful, and a number of sites are running with
it live.

Lets make it available by default, but give the server administrator
a way to easily opt-out of the feature if they run into the same
sort of bug that issue 487 identifies.

Bug: issue 487
Change-Id: I257be95ac93c0775d24fcd36a315f293503febb7
Signed-off-by: Shawn O. Pearce <sop@google.com>
10 files changed