Support newer versions of git

Problem/Motivation

Not sure about the exact version, but git data integrity tests break on git 2.11.0, but they work fine on 2.1.4(testbots use this version, hence it is OK there for now). The output for git show seems to have changed in between.

I am attaching here the a custom diff I generated running the data integrity tests on both versions and adding a debug on VersioncontrolGitRepository::exec().

Proposed resolution

The ideal solution is to stop using git show in favour of a plumbing command. Another option may be to use --raw option

Remaining tasks

Change default synchronizer plugin code.

User interface changes

No,

API changes

No.

Data model changes

No.