Add gitconfig to temporary home directory.

Builds have been failing on Travis CI because of the temporary home
directory: there was no gitconfig, and therefore no user.name or user.email
settings, which prevent commits from being created under some Git versions.
Add the gitsh.gitCommand git-config option

This option is used to specify what command gitsh should use to execute git
commands. When this option is specified using git-config it is overridden by
the command passed using the --git command line option when starting gitsh. The
gitsh.gitCommand variable can also be overridden inside a gitsh session by
using the :set command.

Fixes #111
Add documentation for the gitsh.gitCommand option
Fix style guide violations
Put the dot on the first line
Fix typos
Test git interaction inside a temporary home dir
Remove redundant temporary history file setup
Please hound