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
Merge remote-tracking branch 'upstream/master' into ongoing/latest-jenkins-lts

Conflicts:
	pom.xml