[Question] .git folder file updates for changing head commit

[John Garry <john.garry@huawei.com>]

Hi guys,

I have a question about which files in the .git folder are updated as we 
change the head commit. I could check the codebase myself but prob will 
make a mistake and maybe some expert would be so kind as to just kindly 
tell me.

For building the linux perf tool we use the git head commit id as part 
of the tool version sting. To save time in re-building, the Makefile 
rule has a dependency on .git/HEAD for rebuilding. An alternative 
approach would be to compare git log output to check current versus 
previous build head commit, but that is seen as inefficient time-wise.

The problem is that actions like git cherry-pick and git reset --hard 
HEAD^ may not update .git/HEAD (so don't trigger a rebuild).

Is there some more suitable file(s) which we could use as a dependency? 
 From my limited experimentation, .git/index seems to always update when 
the changing head commit.

Thanks,
john