gitk: treat file names beginning with "|" as relative paths

gitk: treat file names beginning with "|" as relative paths

The Tcl 'open' function has a vary wide interface. It can open files as
well as pipes to external processes. The difference is made only by the
first character of the file name: if it is "|", an process is spawned.

We have a number of calls of Tcl 'open' that take a file name from the
environment in which Gitk is running. Be prepared that insane values are
injected. In particular, when we intend to open a file, do not mistake
a file name that happens to begin with "|" as a request to run a process.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>