Don't provide Pull item if repository is bare

Pull = Fetch + Merge, and Merge needs working tree.

Reproduce Steps:
1. Open Log Message dialog for a bare repository
2. Right click on one commit
=> Pull item shows in context menu

Note:
There is another Pull item appending at line 1938,
but no need to add checking code for it, since it is done at line 1935.

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>