gitk: Clean up file encoding code and add enable/disable option

gitk: Clean up file encoding code and add enable/disable option

This adds an option allowing the user to select whether gitk should
look up per-file encoding settings using git check-attr or not.  If
not, gitk uses the global encoding set in the git config (as reported
by git config --get gui.encoding) for all files, or if that is not
set, then the system encoding.

The option is controlled by a checkbox in the Edit->Preferences
window, and defaults to off for now because git check-attr is so
slow.  When the user turns it on we discard any cached diff file
lists in treediffs, because we may not have encodings cached for
the files listed in those lists, meaning that getblobdiffline will
do it for each file, which will be really really slow.

This adjusts the limit of how many paths cache_gitattr passes to each
instance of git check-attr depending on whether we're running under
windows or not.  Passing only 30 doesn't effectively amortize the
startup costs of git check-attr, but it's all we can do under windows
because of the 32k limit on arguments to a command.  Under other OSes
we pass up to 1000.

Similarly we adjust how many lines gettreediffline processes depending
on whether we are doing per-file encodings so that we don't run for
too long.  When we are, 500 seems to be a reasonable limit, leading
to gettreediffline taking about 60-70ms under Linux (almost all of
which is in cache_gitattr, unfortunately).  This means that we can
take out the update call in cache_gitattr.

This adds a simple cache on [tclencoding].  Now that we get repeated
calls to translate the same encoding, this is useful.

This reindents the new code added in the last couple of commits to
conform to the gitk 4-space indent and makes various other improvements:
use regexp in gitattr and cache_gitattr instead of split + join + regsub,
make gui_encoding be the value from [tclencoding] to avoid having to
do [tcl_encoding $gui_encoding] in each call to get_path_encoding,
and print a warning message at startup if $gui_encoding isn't
supported by Tcl.

Signed-off-by: Paul Mackerras <paulus@samba.org>