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 <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Use mktemp -d to avoid predictable temporary directories

gitk uses a predictable ".gitk-tmp.$PID" pattern when generating
a temporary directory.

Use "mktemp -d .gitk-tmp.XXXXXX" to harden gitk against someone
seeding /tmp with files matching the pid pattern.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Catch mkdtemp errors

105b5d3f ("gitk: Use mktemp -d to avoid predictable temporary
directories") introduced a dependency on mkdtemp, which is not
available on Windows.

Use the original temporary directory behavior when mkdtemp fails.
This makes the code use mkdtemp when available and gracefully
fallback to the existing behavior when it is not available.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
gitk: Add visiblerefs option, which lists always-shown branches

When many branches contain a commit, the branches used to be shown in
the form "A, B and many more", where A, B can be master of current
HEAD. But there are more which might be interesting to always know about.
For example, "origin/master".

The new option, visiblerefs, is stored in ~/.gitk. It contains a list
of references which are always shown before "and many more" if they
contain the commit. By default it is `{"master"}', which is compatible
with previous behavior.

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Merge git://ozlabs.org/~paulus/gitk

* git://ozlabs.org/~paulus/gitk:
  gitk: Add visiblerefs option, which lists always-shown branches
  gitk: Catch mkdtemp errors
  gitk: Use mktemp -d to avoid predictable temporary directories
  gitk: Honor TMPDIR when viewing external diffs