Fix trailing space checker to remove existing trailing spaces

[ksco]

We rely on git diff to do trailing space checks, so to remove the trailing spaces in existing lines that may have been introduced by mistake, deletion lines should be excluded during the check.

Tested with following snippets:

cmake_minimum_required(VERSION 3.0)
project(StringRegexMatchExample)

function(match_string_with_regex input_string regex_pattern)
    string(REGEX MATCH "${regex_pattern}" match_result "${input_string}")
    if(match_result)
        message("Match")
    else()
        message("Not match")
    endif()
endfunction()

set(regex_pattern "\n[+ ][^\n]* \n")

set(input_string "diff\n hello  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n+ \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n-hello  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n-  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n hello\n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n+hello\n-  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n hello\n-  \n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\nhello\n")
match_string_with_regex("${input_string}" "${regex_pattern}")

set(input_string "diff\n\n")
match_string_with_regex("${input_string}" "${regex_pattern}")

Outputs:

Match
Match
Match
Not match
Not match
Not match
Not match
Not match
Not match
Not match
Not match

[derekbruening]

Thank you for improving this check!