pgsql: In basebackup.c, refactor to create read_file_data_into_buffer.

In basebackup.c, refactor to create read_file_data_into_buffer.

This further reduces the length and complexity of sendFile(),
hopefully make it easier to understand and modify. In addition
to moving some logic into a new function, I took this opportunity
to make a few slight adjustments to sendFile() itself, including
renaming the 'len' variable to 'bytes_done', since we use it to represent
the number of bytes we've already handled so far, not the total
length of the file.

Patch by me, reviewed by David Steele.

Discussion: http://postgr.es/m/CA+TgmoYt5jXH4U6cu1dm9Oe2FTn1aae6hBNhZzJJjyjbE_zYig@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c2ba3fdea5938dff96ca32ed7a4a83c8ff5f188d

Modified Files
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src/backend/backup/basebackup.c | 231 +++++++++++++++++++++++-----------------
1 file changed, 133 insertions(+), 98 deletions(-)