gitk: sanitize 'exec' arguments: simple cases

gitk: sanitize 'exec' arguments: simple cases

Tcl 'exec' assigns special meaning to its argument when they begin with
redirection, pipe or background operator. There are a number of
invocations of 'exec' which construct arguments that are taken from the
Git repository or a user input. However, when file names or ref names
are taken from the repository, it is possible to find names with have
these special forms. They must not be interpreted by 'exec' lest it
redirects input or output, or attempts to build a pipeline using a
command name controlled by the repository.

Introduce a helper function that identifies such arguments and prepends
"./" to force such a name to be regarded as a relative file name.

Convert those 'exec' calls where the arguments can simply be packed
into a list.

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