Performance issue of 'git branch'

Hi,

When I run 'git branch' in the linux-2.6 repo I think it takes
too long to finish (with cold cache):

[mafra@Pilar:linux-2.6]$ time git branch
  27-stable
  28-stable
  29-stable
  30-stable
  dev-private
* master
  option
  sparse
  stern
0.00user 0.05system 0:05.73elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (209major+1380minor)pagefaults 0swaps

This is with git 1.6.4.rc1.10.g2a67 and the kernel is 2.6.31-rc3+. The
machine is a 64bit Vaio laptop which is 1+ year old (so it is not "slow").

Repeating the command a second time takes basically zero seconds, but
this is more or less what I would expect in the first time too.

I use git to track linux-2.6 for 2 years now, and I remember that
'git branch' is slow for quite some time, so it is not a regression
or something. It is just now that I took the courage to report this
small issue.

I did a 'strace' and this is where it spent most of the time:

1248301060.654911 open(".git/refs/heads/sparse", O_RDONLY) = 6
1248301060.654985 read(6, "60afdf6a4065a170ad829b4d79a86ec0"..., 255) = 41
1248301060.655056 read(6, "", 214)      = 0
1248301060.655116 close(6)              = 0
1248301060.680754 lstat(".git/refs/heads/stern", 0x7fff80bfa8d0) = -1 ENOENT (No such file or directory)
1248301064.018491 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
1248301064.018641 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f409ffa7000
1248301064.018722 write(1, "  27-stable\33[m\n", 15) = 15

I don't know why .git/refs/heads/stern does not exist and why it takes
so long with it. That branch is functional ('git checkout stern' succeeds),
as well as all the others. But strangely .git/refs/heads/ contains only

[mafra@Pilar:linux-2.6]$ ls .git/refs/heads/
dev-private  master  sparse

which, apart from "master", are the last branches that I created.

I occasionally run 'git gc --aggressive --prune" to optimize the repo,
but other than that I don't do anything fancy, just 'pull' almost
every day and 'bisect' (which is becoming a rare event now :-)

So I would like to ask what should I do to recover the missing files
in .git/refs/heads/ (which apparently is the cause for my issue) and
how I can avoid losing them in the first place.

Also, is there a way to "fix" the 4-secs pause in that lstat() in
case the files in .git/refs/heads/ get lost again?

Thanks in advance,
Carlos