show: honor --textconv for blobs

Currently, "diff" and "cat-file" for blobs honor "--textconv" options
(with the former defaulting to "--textconv" and the latter to
"--no-textconv") whereas "show" does not honor this option, even though
it takes diff options.

Make "show" on blobs behave like "diff", i.e. honor "--textconv" by
default and "--no-textconv" when given.
cat-file: do not die on --textconv without textconv filters

When a command is supposed to use textconv filters (by default or with
"--textconv") and none are configured then the blob is output without
conversion; the only exception to this rule is "cat-file --textconv".

Make it behave like the rest of textconv aware commands.
t7008: demonstrate behavior of grep with textconv

Currently, "git grep" does not honor any textconv filters. Demonstrate
this in the tests.
grep: allow to use textconv filters

Recently and not so recently, we made sure that log/grep type operations
use textconv filters when a userfacing diff would do the same:

ef90ab6 (pickaxe: use textconv for -S counting, 2012-10-28)
b1c2f57 (diff_grep: use textconv buffers for add/deleted files, 2012-10-28)
0508fe5 (combine-diff: respect textconv attributes, 2011-05-23)

"git grep" currently does not use textconv filters at all, that is
neither for displaying the match and context nor for the actual grepping.

Introduce an option "--textconv" which makes git grep use any configured
textconv filters for grepping and output purposes. It is off by default.

Signed-off-by: Jeff King <peff@peff.net>
grep: honor --textconv for the case rev:path

Make "grep" honor the "--textconv" option also for the object case, i.e.
when used with an argument "rev:path".
git grep: honor textconv by default

Currently, "git grep" does not honor textconv settings by default.
Make it honor them by default just like "git log --grep" does.