gitk: Put temporary directory inside .git

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: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com> Signed-off-by: Paul Mackerras <paulus@samba.org>