gitk fails to show diffs when browsing a read-only repository.
This is due to gitk's assumption that the current directory is always
writable.
Teach gitk to honor either the GITK_TMPDIR or TMPDIR environment
variables. This allows users to override the default location
used when writing temporary files.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Paul Mackerras <paulus@samba.org>