gitk: Do not hard-code "encoding" in attribute lookup functions

gitk: Do not hard-code "encoding" in attribute lookup functions

Commit 39ee47e (Clean up file encoding code and add enable/disable option,
2008-10-15) rewrote the attribute lookup functions gitattr and
cache_gitattr, but in the process hard-coded the attribute name "encoding"
instead of using the functions' parameters. This fixes it.

This is not a serious regression because currently all callers look only
for "encoding".

Further note that this fix assumes that future callers will not pass an
attribute name that contains regex special characters.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>