gitk: be prepared to be run in a bare repository

784b7e2f ("gitk: Fix "External diff" with separate work tree",
2011-04-04) added an unconditional call to "git rev-parse
--show-toplevel" to set up a global variable quite early in the
course of the program, so that the location of the working tree can
later be known if/when the user chooses to run the external diff via
the external_diff_get_one_file proc.  Before that change, the
external diff code used to assume that the parent directory of ".git"
directory is the top-level of the working tree.

Recent versions of git however notices that "rev-parse --show-toplevel"
executed in a bare repository is an error, which makes gitk stop,
even before the user could attempt to run external diff.

Use the gitworktree helper introduced in 65bb0bda ("gitk: Fix the
display of files when filtered by path", 2011-12-13), which is
prepared to see failures from "rev-parse --show-toplevel" and other
means it tries to find the top-level of the working tree instead to
work around this issue.  The resulting value in $worktree global,
when run in a bare repository, is bogus, but the code is not
prepared to run external diff correctly without a working tree
anyway ;-)

[paulus@ozlabs.org - folded in fix from Eric Sunshine]

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

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