Bug: git-describe abbrev documentation mentions wrong default

[Martin Mosegaard Amdisen <martinmosegaard@xxxxxxxxx>]

The documentation for the "git describe --abbrev" flag says that the default value is 7 hexadecimal digits:

https://github.com/git/git/blob/master/Documentation/git-describe.txt#L63 and

https://git-scm.com/docs/git-describe

I have experienced that sometimes I see 7 digits, but other times more.

The behavior seems to have changed in git 2.11.0 ("default abbreviation length"):

https://github.com/git/git/blob/aeddbfdfa48443c034a9b28b10dfddf2f71b907f/Documentation/RelNotes/2.11.0.txt

I was able to trigger different behavior using a bash script like the one below:

```sh

#!/bin/bash

DIR=test-git-describe

rm -rf ${DIR}

mkdir ${DIR}

cd ${DIR}

git init

# One-time test results I have seen:

# 10 tags, 10 * 10 commits: 7 hex digits

# 100 tags, 100 * 100 commits: 8 hex digits

FILE=foo

touch ${FILE}

for i in `seq 1 10`; do

    echo ${i} >> ${FILE}

    git add ${FILE}

    git commit -m "Tag update number ${i} to ${FILE}"

    git tag -m "Version 1.0.${i}" -a 1.0.${i}

    for j in `seq 1 10`; do

        echo ${i}-${j} >> ${FILE}

        git add ${FILE}

        git commit -m "Update number ${i}-${j} to ${FILE}"

    done

done

```