test git version string

The git version string is used in the wild, and in git gui,
for capability checking purposes. Test the string format
and version X.Y.Z number.

The version string shall be:
* a single line
* less that 80 characters
* start with `git version `
* with a following numeric X.Y.Z version.

The remaining characters are undefined.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Doc: State the exact git version string

Provide the regular expression that matches the
'git --version' strings.

Scripts in the wild, including 'git gui', check the git version.

Signed-off-by: Philip Oakley <philipoakley@iee.org>