pack-write: pass hash_algo to internal functions

pack-write: pass hash_algo to internal functions

The internal functions `write_rev_trailer()`, `write_rev_trailer()`,
`write_mtimes_header()` and write_mtimes_trailer()` use the global
`the_hash_algo` variable to access the repository's hash function. Pass
the hash_algo down from callers, all of which already have access to the
variable.

This removes all global variables from the 'pack-write.c' file, so
remove the 'USE_THE_REPOSITORY_VARIABLE' macro.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>