Execute the 'git ...' rather than 'git-...'

Since many commands are now built into the git binary, it doesn't make
sense to run the git-... scripts. The patch modifies the GRun class to
pass the 'git' file name together with the command arguments. It also
modifies all the other calls to git-...

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>