pgsql: Code review/prettification for generic_xlog.c.

Code review/prettification for generic_xlog.c.

Improve commentary, use more specific names for the delta fields,
const-ify pointer arguments where possible, avoid assuming that
initializing only the first element of a local array will guarantee
that the remaining elements end up as we need them.  (I think that
code in generic_redo actually worked, but only because InvalidBuffer
is zero; this is a particularly ugly way of depending on that ...)

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/db03cf375d602e417eda6b7a55eead91618e1398

Modified Files
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src/backend/access/transam/generic_xlog.c | 159 +++++++++++++++++-------------
1 file changed, 92 insertions(+), 67 deletions(-)