amend remaining usage strings according to style guide

amend remaining usage strings according to style guide

Usage strings for git (sub)command flags has a style guide that
suggests - first letter should not capitalized (unless required)
and it should skip full-stop at the end of line. But there are
some files where usage-strings do not follow the above mentioned
guide.

Amend the usage strings that don't follow the style convention/guide.

Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>