When running "External diff" from gitk, the "from" and "to" files will
first be copied into a directory that is currently
".git/../.gitk-tmp.$pid".  When gitk is closed, the directory is
deleted. When the work tree is not at ".git/.." (which is supported
since the previous commit), that directory may not even be git-related
and it does not seem unlikely that permissions may not allow the
temporary directory to be created there.  Move the directory inside
.git instead.

This introduces a regression in the case that the .git directory
is readonly, but .git/.. is writeable.
Signed-off-by: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>