fix protocol check for cli's rev-parse --resolve-geogit-dir
fix functional test errors introduced at 00c94da
'peel' the target Ref before updating HEAD when checkout --force is used.

CheckoutOp was updating HEAD to point to HEAD when force == true, leaving the repository in an inconsistent
state. This patch adds a precondition check to FileRefDatabase.putSymRef(...) to avoid self refs, and makes
sure CheckoutOp 'peels' the target Ref before updating HEAD.
Minor formatting improvements in documentation.

Added space after prompt character in shell examples
Replaced *git* by Git
Extended log command with additional options and improved behaviour
Added a ParseTimestamp operation with support for different syntaxes
Added support for parsing partial representations of the null id

Added mention to null id in documentation
Merge pull request #253 from volaya/null_id

Added support for parsing partial representations of the null id
Minor changes to pull request.

Fixed compilation error in man page.
Fixed typo in history manual page.
Removed unneeded warning suppression.
Fixed test failure in LogOpPerformanceTest.
Merge branch 'log_improvements'