Update to latest released JaCoCo 0.7.0

Previous release is a year old and does not support Java 8.
Upgrade to JGit 3.3.2

Includes a bug fix to only reset credentials after multiple attempts.

See https://git.eclipse.org/c/jgit/jgit.git/commit/?h=stable-3.3&id=a53a18e988714cd87934d1a7e29b3331b37542a8

No automated tests currently evaluate authenticated access, so it is
not clear if that bug fix is relevant or not.
Use interesting UTF-8 characters in the "clean" test

Attempting to confirm JENKINS-20410 and JENKINS-22434.
Add more interesting characters, correct encoding mistake made earlier
Specify file content encoding explicitly in GitAPITestCase
Add a composed Unicode character to clean test
More attempts to assure unicode file names are properly cleaned
Another test of [JENKINS-22434], shows failure if LANG=C
Use --progress only when it's supported

This option is relatively new in 1.7.1.
See
http://stackoverflow.com/questions/21735978/jenkins-git-fetch-fails-with-unknown-option-progress
for the detective work
Warn if git version does not support credentials

[JENKINS-21887] The git credentials support was not added until git 1.7.9.

This commit will only provide a warning in the log file, but will
still attempt to execute the credentials operations.
log executed command on build console to help user diagnose issues
[Fix JENKINS-22039] Use new temp dir for remote polling credentials check

The default behavior was to create a new git directory in the system
temporary directory.  Since git repositories apply to all child
directories, that had the bad result of making the system temporary
directory appear as though it were under git control.

That also caused spurious automated test failures in the git-client
unit tests.
[maven-release-plugin] prepare release git-client-1.8.1
[maven-release-plugin] prepare for next development iteration
Update README.md

Simplify phrasing in the README
Add tests of Revision object
Cover a few more of the Revision branches / statements
Sanitize whitespace in RevisionTest
Add test of GitException with no message string
Add test of GitLockFailedException