git: Add option for no_config in git shell

[jerry-skydio]

This causes git commands to run without considering any
system or user configuration. In some cases it's easier to
ban all overrides rather than have to override things on
a flag by flag basis.

[jerry-skydio]

Reviews in this chain:
└#188 git: Add option for no_config in git shell
　└#187 git: Use merge-tree in patchsets

[jerry-skydio]

#	head	base	diff	date	summary
0	1d315c52	740c5cdb	diff	Jul 3 13:56 PM	1 file changed, 16 insertions(+), 2 deletions(-)
1	cb8fa697	740c5cdb	diff	Jul 3 14:08 PM	1 file changed, 12 insertions(+), 6 deletions(-)
2	64fe866f	740c5cdb	diff	Jul 8 16:07 PM	0 files changed