Use long constant in expression computing git version

Low risk change that silences a Coverity warning.
Don't use regular expression replacer for fixed string in git version parsing

Removing "msysgit." from the version string does not require the single 
character wildcard at the end of the string.
Fix Windows longpath tests on one problematic Win7 computer

The changes are only cosmetic, and the root reason for the test failures
is still unclear, since the same tests pass on other Windows 7 machines
and on onther Windows machines.
Use all credential tests if TEST_ALL_CREDENTIALS is true

Test credentials depend on a local file containing credentials to be tested

Most developers and most Jenkins jobs will not define TEST_ALL_CREDENTIALS
Confirmed that JGit 3.5.2 still prunes more than CliGit

Prune during fetch is currently disabled for JGit because JGit behaves
differently than command line git when pruning during fetch.  The unit
test test_fetch_with_prune() in GitAPITestCase shows the difference.
Merge remote-tracking branch 'upstream/master' into master-fix-javadoc-for-java-8