t3701-add-interactive: tighten the check of trace output

t3701-add-interactive: tighten the check of trace output

The test 'add -p does not expand argument lists' in
't3701-add-interactive.sh', added in 7288e12cce (add--interactive: do
not expand pathspecs with ls-files, 2017-03-14), checks the GIT_TRACE
of 'git add -p' to ensure that the name of a tracked file wasn't
passed around as argument to any of the commands executed as a result
of undesired pathspec expansion.  This check is done with 'grep' using
the filename on its own as the pattern, which is too loose a pattern,
and would match any occurrences of the filename in the trace output,
not just those as command arguments.  E.g. if a developer were to
litter the index handling code with trace_printf()s printing, among
other things, the name of the just processed cache entry, then that
pattern would mistakenly match these as well, and would fail the test.

Tighten this 'grep' pattern to only match trace lines that show the
executed commands.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>