Git without morning coffee

Is this my lack of morning coffee, or git's:

git rev-parse origin/next origin/pu
7931f38a2fd882b0f75a4d6f0eb60c3b1b094178
edf014b39ded8ee8a67f2a0cd8d7926b33f3d578
# OK

git checkout origin/next
HEAD is now at 7931f38... Merge branch 'master' into next
# OK

git log --oneline --no-walk ":/Merge branch 'jk/generation-numbers' into pu"
0ac76f9 Merge branch 'jk/generation-numbers' into pu
# Sure it is.

git merge ":/Merge branch 'jk/generation-numbers' into pu"
fatal: ':/Merge branch 'jk/generation-numbers' into pu' does not point
to a commit
# Huh?

git rev-parse ":/Merge branch 'jk/generation-numbers' into pu"
0ac76f96a94a46bce7ec9dcc6c67abcf9d36adac
# It does point to a commit!

git merge $(git rev-parse ":/Merge branch 'jk/generation-numbers' into pu")
error: addinfo_cache failed for path 't/t7810-grep.sh'
Performing inexact rename detection: 100% (91476/91476), done.
error: addinfo_cache failed for path 't/t7810-grep.sh'
error: addinfo_cache failed for path 'gitweb/static/gitweb.js'
error: addinfo_cache failed for path 'gitweb/static/gitweb.css'

## hundreds more

error: addinfo_cache failed for path 't/t7810-grep.sh'
error: addinfo_cache failed for path 'gitweb/static/js/blame_incremental.js'
Auto-merging cache.h
Merge made by the 'recursive' strategy.


Double Huh? Finally, the merge succeeds, despite thy myriad of errors.

I mean, I'm merging a commit from origin/pu to origin/next when the
latter is basically contained in the former (except for some merge
commits). And what are these addinfo_cache errors?

git --version
git version 1.7.7.rc0.328.g9d6c7

which is last week's next with a few additions which I've been carrying
along for weeks. Stumped.

Michael