t4030: demonstrate behavior of show with textconv

"git show <commit>" honors the --textconv option while "git show <blob>"
does not. Demonstrate this in the test.

Since the current behavior is supposed to stay as is, we expect the
default for "git show <blob>" to remain --no-textconv.
diff_opt: track whether flags have been set explicitly

The diff_opt infrastructure sets flags based on defaults and command
line options. Currently, it is impossible to detect whether a flag has
been set as a default or on explicit request.

Amend the structure so that this detection is possible:

 * There is an extra "opt->touched_flags" that keeps track of all
   the fields that have been touched by DIFF_OPT_SET and
   DIFF_OPT_CLR;

 * You may continue setting the default values to the flags, like
   commands in the "log" family do in cmd_log_init_defaults(), but
   after you finished setting the defaults, you clear the
   touched_flags field;

 * And then you let the usual callchain call diff_opt_parse(),
   allowing the opt->flags be set or unset, while keeping track of
   which bits the user touched;

 * There is an optional callback "opt->set_default" that is called
   at the very beginning to let you inspect touched_flags and
   update opt->flags appropriately, before the remainder of the
   diffcore machinery is set up, taking the opt->flags value into
   account.
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" and
"--no-textconv" when given. The default for blobs remains unchanged
("--no-textconv").
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, with nor
without --textconv. Demonstrate this in the tests.

The default is expected to remain unchanged.
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,
even when requested by --textconv.

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".