t4108: remove git command upstream of pipe

t4108: remove git command upstream of pipe

Before, the output of `git diff HEAD` would always be piped to
sanitize_conflicted_diff(). However, since the Git command was upstream
of the pipe, in case the Git command fails, the return code would be
lost. Rewrite into separate statements so that the return code is no
longer lost.

Since only the command `git diff HEAD` was being piped to
sanitize_conflicted_diff(), move the command into the function and rename
it to print_sanitized_conflicted_diff().

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>