gitk: Implement batch lookup and caching of encoding attrs

gitk: Implement batch lookup and caching of encoding attrs

When the diff contains thousands of files, calling git-check-attr once
per file is very slow.  With this patch gitk does attribute lookup in
batches of 30 files while reading the diff file list, which leads to a
very noticeable speedup.

It may be possible to reimplement this even more efficiently, if
git-check-attr is modified to support a --stdin-paths option.
Additionally, it should quote the ':' character in file paths, or
provide a more robust way of column separation.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Paul Mackerras <paulus@samba.org>