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Add C comment about problems with CHAR() space trimming

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Original file line number	Diff line number	Diff line change
`@@ -846,6 +846,18 @@ bpcharcmp(PG_FUNCTION_ARGS)`
`846`	`846`	`len2;`
`847`	`847`	`intcmp;`
`848`	`848`
	`849`	`+/*`
	`850`	`+ * Trimming trailing spaces off of both strings can cause a string`
	`851`	`+ * with a character less than a space to compare greater than a`
	`852`	`+ * space-extended string, e.g. this returns false:`
	`853`	`+ *SELECT E'ab\n'::CHAR(10) < E'ab '::CHAR(10);`
	`854`	`+ * even though '\n' is less than the space if CHAR(10) was`
	`855`	`+ * space-extended. The correct solution would be to trim only`
	`856`	`+ * the longer string to be the same length of the shorter, if`
	`857`	`+ * possible, then do the comparison. However, changing this`
	`858`	`+ * might break existing indexes, breaking binary upgrades.`
	`859`	`+ * For details, see http://www.postgresql.org/message-id/CAK+WP1xdmyswEehMuetNztM4H199Z1w9KWRHVMKzyyFM+hV=zA@mail.gmail.com`
	`860`	`+ */`
`849`	`861`	`len1=bcTruelen(arg1);`
`850`	`862`	`len2=bcTruelen(arg2);`
`851`	`863`

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