Use an unreachable commit ID instead of a tag

[pjw91]

For git v1.7.1 (eg. on CentOS 6), a shallow clone achieved by git clone --depth 1 clones not only the latest commit on master branch but also all branches and all tags, which makes the tag jq-1.0 available.

As a result, the shallow-clone-detecting command passes and the git describe is incorrectly called.

This patch replaces the tag jq-1.0 with eca89ac, which is the initial commit.

[coveralls]

Coverage increased (+0.4%) to 85.071% when pulling ca21a75 on mail6543210:fix-shallow into 79ece35 on stedolan:master.

[itchyny]

As of git v1.7.10 (April 2012), using --depth=1 implies --single-branch flag so tags are not fetched (ref: RelNotes/1.7.10.txt#L80-L82, https://lkml.org/lkml/2012/3/28/418). So the version shows master-[commit sha] right after a shallow clone of the repository. But doing git fetch --tags after the shallow clone, autoreconf fails with fatal: No tags can describe error. In this situation, git rev-parse --verify -q eca89ac resolves the full commit sha (not sure why) so the proposed change does not fix the error. We can use git describe --tags --match 'jq-*' (or just git describe --tags for simplicity) for this check IMO. Here are the related commits: f7bf9a8, 6878123.