Restore use of 'help.format' configuration property in 'git help'

Restore use of 'help.format' configuration property in 'git help'

Commit 1cc8af0 "help: use HTML as the default help format on Windows"
lost the ability to make use of the help.format config value by forcing
the use of a compiled in default if no command-line argument was provided.
This commit restores the use of the help.format value if one is
available, overriding the compiled default.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>