gitk: Honor TMPDIR when viewing external diffs
gitk: Honor TMPDIR when viewing external diffs

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 <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>
Signed-off-by: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git