bug#10923: git-version-gen reports wrong version after merge

[BT Templeton]

After merging a branch, version.test is failing, apparently because
"git-version-gen" is generating an incorrect version number:

#+begin_example
olor:~/src/guile(master)% ./build-aux/git-version-gen .tarball-version
2.0.5.351-1c3ec
olor:~/src/guile(master)% grep = GUILE-VERSION | head -3
GUILE_MAJOR_VERSION=2
GUILE_MINOR_VERSION=1
GUILE_MICRO_VERSION=0
olor:~/src/guile(master)% git describe
v2.0.5-351-g1c3ecea
olor:~/src/guile(master)% git describe --match v2.1\*
v2.1.0-532-g1c3ecea
#+end_example

I assume that the v2.0.5 tag is simply closer in the commit graph than
v2.1.0 (if git is looking at both parents of the merge commit). I'm not
sure what the correct solution is, but this page describes some possible
workarounds:
<http://www.xerxesb.com/2010/git-describe-and-the-tale-of-the-wrong-commits/>

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