[pgm] Add option --heads for ls-remote

TODO: find a way to use option -h which is already captured by
TextBuiltin's option --help which also uses the alias -h.

Bug: 444072
Change-Id: Ie66584c2fc7fc224014a43cf928547703dd9d213
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
[pgm] Add option --tags for ls-remote

Bug: 444072
Change-Id: I52b470924609fc201e078d9d232aad257506728d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Fix junit tests under windows when the platform is explicitly changed

SystemReader used a chached ObjectChecker which was instantiated only
once. But in case of unit tests where we can change the platform
dynamically (e.g. MockSystemReader.setWindows()) this is wrong and
caused DirCacheCheckoutMaliciousPathTest.
testMaliciousAbsoluteCurDrivePathWindowsOnUnix() to fail. This change
allows 
user of SystemReader to force the creation of a new ObjectChecker. 
MockSystemReader.setWindows() and .setUnix() make use of this feature.

Change-Id: I87458d1dc63c1f5c18979f972b1c1f0d670a9ed8
Merge "[pgm] Add option --heads for ls-remote" into stable-3.6
Merge "[pgm] Add option --tags for ls-remote" into stable-3.6
Merge "Fix junit tests under windows when the platform is explicitly changed" into stable-3.6