t6050: avoid pipes with upstream Git commands

t6050: avoid pipes with upstream Git commands

In pipes, the exit code of a chain of commands is determined by
the final command. In order not to miss the exit code of a failed
Git command, avoid pipes instead write output of Git commands
into a file.
For better debugging experience, instances of "grep" were changed
to "test_grep". "test_grep" provides more context in case of a
failed "grep".

Signed-off-by: Chizoba ODINAKA <chizobajames21@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>