checkpatch: add new exception to repeated word check

Recently, commit 4f6ad8aa1eac ("checkpatch: move repeated word test")
moved the repeated word test to check for more file types. But after
this, if checkpatch.pl is run on MAINTAINERS, it generates several
new warnings of the type:

  WARNING: Possible repeated word: 'git'

For example:

  WARNING: Possible repeated word: 'git'
  +T:	git git://git.kernel.org/pub/scm/linux/kernel/git/rw/uml.git

So, the pattern "git git://..." is a false positive in this case.

There are several other combinations which may produce a wrong warning
message, such as "@size size", ":Begin begin", etc.

Extend repeated word check to compare the characters before and after
the word matches.

If there is a non whitespace character before the first word or a non
whitespace character excluding punctuation characters after the second
word, then the check is skipped and the warning is avoided.

Also add case insensitive word matching to the repeated word check.

Link: https://lore.kernel.org/linux-kernel-mentees/81b6a0bb2c7b9256361573f7a13201ebcd4876f1.camel@perches.com/
Link: https://lkml.kernel.org/r/20201017162732.152351-1-dwaipayanray1@gmail.com


Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Cc: Aditya Srivastava <yashsri421@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>