grep: implement -x for fixed strings

[mknos]

• Standards document mentions that option -x should work with fixed strings [1]
• This version didn't allow -x with -F; add it now for better compliance with standard
• I have a small test file with some C code
• test1: "perl grep int a.c" --> matches "int\n" and "printf(...);\n"
• test2: "perl grep -i Int a.c" --> same match as test1
• test3: "perl grep -iv Int a.c" --> matches all except "int\n" and "printf(...);\n"
• test4: "perl grep -v Int a.c" --> matches all lines because Int doesn't appear at all
• test5: "perl grep -F -x int a.c" --> matches only "int\n"
• test6: "perl grep -Fxi Int a.c" --> same match as test5
• test7: "perl grep -Fxiv Int a.c" --> matches all except "int\n"
• test8: "perl grep -Fxv Int a.c" --> matches all lines because Int doesn't appear at all

1. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html

[coveralls]

Pull Request Test Coverage Report for Build 10995604813

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 73.069%

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Totals
Change from base Build 10982053865:	-0.7%
Covered Lines:	350
Relevant Lines:	479

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