gitk: Tcl9 doesn't expand ~, use $env(HOME)

gitk looks for configuration files under $(HOME)/.., and uses the
typical shortcut formats to find this, e.g., ~/.config/. This relies
upon Tcl expanding such constructs to replace ~ with $(HOME). But, Tcl 9
has stopped doing that for various reasons, and now supplies [file
tildeexpand ...] to perform this expansion.

There are a very few places that need this expansion, and all must be
modified regardless of approach taken.

POSIX specifies that $HOME be defined at the time of login, and both
Cygwin and MSYS (underlying git for windows) set this variable. Tcl8
uses the POSIX defined pwnam to look up the underlying database record
on Unix, but will get the same result as using $HOME on any POSIX
compliant system. On Windows, Tcl just accesses $HOME, falling back to
other environment variables if $HOME is not set. Git for Windows has
$HOME defined by MSYS, so this works just as on the others.

As $env(HOME) works in Tcl 8 and 9, while anything using [file
tildeexpand ... ] will not, let's use the simpler approach as doing so
adds no lines of code.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>