log -S with evil merges

[Jonathan Nieder <jrnieder@gmail.com>]

Hi,

I was interested to know the origin of the UE_ALWAYS et al flags in
builtin/reflog.c, so I asked git:

 ; git log --oneline -S'UE_ALWAYS' --follow builtin/reflog.c
 81b50f3 Move 'builtin-*' into a 'builtin/' subdirectory
 4264dc1 git reflog expire

Ok, clearly it was not introduced in the builtin/ move.  I guess the
UE_ flags were part of the original ‘reflog’ implementation.  Right?

 ; git show 4264dc1 | grep UE_ALWAYS
 ; git show 4264dc1:builtin-reflog.c | grep UE_ALWAYS
 ;

No, that’s not it.  At this point I decided -S is broken and just did
a grep through log -p, to find that apparently this symbol was
introduced in builtin-reflog.c with v1.7.2-rc0~122^2 (reflog
--expire-unreachable: special case entries in "HEAD" reflog,
2010-04-09).

So why is log confused?  As far as log -S knows, because of the
builtin/ move, commit v1.7.2-rc0~122 was an evil merge that produced
the UE_ flags in builtin/reflog.c out of thin air.  It should be
blaming that commit, then.  With a little coaxing, it does.

 ; git log --format=oneline \
	--first-parent -m -S'UE_ALWAYS' --follow builtin/reflog.c |
   git name-rev --tags --stdin |
   cut -d' ' -f2-
 (tags/v1.7.2-rc0~122) Merge branch 'jc/maint-no-reflog-expire-unreach-for-head'
 (tags/v1.7.1-rc0~76) Merge branch 'lt/deepen-builtin-source'
 (tags/v1.5.0-rc1~188) Merge branch 'jc/fsck-reflog'
 ;

The first is the culprit, the second and third the file’s renaming and
introduction[1].

The more logical

 ; git log --oneline -c -S'UE_ALWAYS' --follow builtin/reflog.c

does not work --- it mentions _all_ merges.  Why?

[1] I don’t know why ‘log -S --follow’ should feel the need to point
these out, but that’s a different story.