git tag --contains <commit> -n=1 ?

Hi,
I'm thinking how to find the oldest tag containing a specified commit.
Eg.
$ cd /usr/src/linux-linus/Documentation
$ for i in *.txt; do
for> echo $i
for> hash=`git log -1 --format=format:%H -- $i`
for> git tag --contains $hash -l 'v2.6.[0-9][0-9]'|head -1
for> done
DMA-API.txt
v2.6.31
DMA-ISA-LPC.txt
v2.6.20
DMA-attributes.txt
v2.6.27
...

Here I wonder whether "head -1" is generally correct or not when I want
the oldest tag.
Moreover, as "git tag --contains ..." takes considerable time, I will
be happy if I can set the maximum number in searching tags containing
a commit.  Or are there already some (better) ways to achieve this?

Any advice is welcome.

Thanks,
Kai