Revision 1759399

Fix the excessive amount of read and seek syscalls in various fsfs operations
that read from disk.

The idea behind the fix is that we provide a special svn_stream_readline()
implementation for APR file-based streams that handles a common case of
LF ("\n") end-of-line sequence with apr_file_gets().

For example, this patch turns the following sequence ...

    open("/repo/db/revs/22.pack/manifest", O_RDONLY|O_CLOEXEC) = 4
    read(4, "0\n3304\n7139\n12288\n13981\n37151\n59"..., 4096) = 4096
    read(4, "5140\n20998974\n21002732\n21004670\n"..., 4096) = 4096
    lseek(4, 4020, SEEK_SET)                = 4020
    read(4, "18102627\n18139220\n18141921\n18147"..., 4096) = 4096
    read(4, "2535219\n52537641\n52550290\n525536"..., 4096) = 584
    lseek(4, 8043, SEEK_SET)                = 8043
    read(4, "51452518\n51454318\n51456275\n51457"..., 4096) = 657
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    read(4, "", 4096)                       = 0
    close(4)                                = 0

...into this:

    open("/repo/db/revs/22.pack/manifest", O_RDONLY|O_CLOEXEC) = 4
    read(4, "0\n3304\n7139\n12288\n13981\n37151\n59"..., 4096) = 4096
    read(4, "5140\n20998974\n21002732\n21004670\n"..., 4096) = 4096
    read(4, "131\n52730772\n52744939\n52748273\n5"..., 4096) = 508
    read(4, "", 4096)                       = 0
    close(4)

* subversion/libsvn_subr/stream.c
  (readline_apr_lf): New.
  (readline_apr_generic): New, factored out from ...
  (readline_apr_handler): ...here.  Call the apr_file_gets()-based routine
   when we're looking for an LF end-of-line sequence to avoid unnecessary
   syscalls.

* subversion/tests/libsvn_subr/stream-test.c
  (test_stream_readline_file): New helper for ...
  (test_stream_readline_file_lf, test_stream_readline_file_crlf): ...these
   new tests that cover svn_stream_readline() behavior for LF and CRLF
   terminated lines.
  (test_funcs): Add new tests.