gitk: Check git version before using --textconv flag

gitk: Check git version before using --textconv flag

Commit 785b7535 ("gitk: use --textconv to generate diff text")
added the --textconv flag to the git diff commands used to
display commit diffs.  Since some people use newer gitk with older
git installations, this adds a check on the git version to check
that it understands --textconv before using it.

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