log: rename "tweak" helpers

log: rename "tweak" helpers

The revision walking API allows the callers to tweak its
configuration at the last minute, immediately after all the revision
and pathspec parameters are parsed from the command line but before
the default actions are decided based on them, by defining a "tweak"
callback function when calling setup_revisions().  Traditionally,
this facility was used by "git show" to turn on the patch output
"-p" by default when no diff output option (e.g.  "--raw" or "-s" to
squelch the output altogether) is given on the command line, and
further give dense combined diffs "--cc" for merge commits when no
option to countermand it (e.g. "-m" to show pairwise patches).

Recently, "git log" started using the same facility, but we named
the callback function "default_follow_tweak()", as if the only kind
of tweaking we would want for "git log" will forever be limited to
turning "--follow" on by default when told by a configuration
variable.  That was myopic.

Rename it to more generic name "log_setup_revisions_tweak()", and
match the one used by show "show_setup_revisions_tweak()".

Signed-off-by: Junio C Hamano <gitster@pobox.com>