Add option to allow using JDK's SHA1 implementation

Add option to allow using JDK's SHA1 implementation

The change If6da9833 moved the computation of SHA1 from the JVM's
JCE to a pure Java implementation with collision detection.
The extra security for public sites comes with a cost of slower
SHA1 processing compared to the native implementation in the JDK.

When JGit is used internally and not exposed to any traffic from
external or untrusted users, the extra cost of the pure Java SHA1
implementation can be avoided, falling back to the previous
native MessageDigest implementation.

Bug: 580310
Change-Id: Ic24c0ba1cb0fb6282b8ca3025ffbffa84035565e