diff: add flag to indicate textconv was set via cmdline

diff: add flag to indicate textconv was set via cmdline

git-show is unique in that it wants to use textconv by default except
for when it is showing blobs.  When asked to show a blob, show doesn't
want to use textconv unless the user explicitly requested that it be
used by providing the command line flag '--textconv'.

Currently this is done by using a parallel set of 'touched' flags which
get set every time a particular flag is set or cleared.  In a future
patch we want to eliminate this parallel set of flags so instead of
relying on if the textconv flag has been touched, add a new flag
'TEXTCONV_SET_VIA_CMDLINE' which is only set if textconv is set to true
via the command line.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>