LibGit: Enable fetch function

but all call back is empty

Signed-off-by: Frank Li <lznuaa@gmail.com>
Libgit2: Fetch can count update

but update tip fail when run like "git fetch url branch"

Signed-off-by: Frank Li <lznuaa@gmail.com>
libgit2 clone: Show ref update information after fetch

Signed-off-by: Frank Li <lznuaa@gmail.com>
GitProgressDlg: Add context menu support to open file or show log

Signed-off-by: Frank Li <lznuaa@gmail.com>