new GIT-VERSION-GEN with old GIT

On Debian, git 1.4.4 is still shipped in the stable distribution.

Since e5fc9a0aea2c3c49829b5cdf499339e5c759706b, simply running make in
a git checkout yields to an error message.

The error is in GIT-VERSION-GEN:
| + git diff-index --quiet HEAD
| usage: git-diff-index [-m] [--cached] [<common diff options>] <tree-ish> [<path>...]
| common diff options:
|   -z            output diff-raw with lines terminated with NUL.
|   -p            output patch format.
|   -u            synonym for -p.
|   --patch-with-raw
|                 output both a patch and the diff-raw format.
|   --stat        show diffstat instead of patch.
|   --numstat     show numeric diffstat instead of patch.
|   --patch-with-stat
|                 output a patch and prepend its diffstat.
|   --name-only   show only names of changed files.
|   --name-status show names and status of changed files.
|   --full-index  show full object name on index lines.
|   --abbrev=<n>  abbreviate object names in diff-tree header and diff-raw.
|   -R            swap input file pairs.
|   -B            detect complete rewrites.
|   -M            detect renames.
|   -C            detect copies.
|   --find-copies-harder
|                 try unchanged files as candidate for copy detection.
|   -l<n>         limit rename attempts up to <n> paths.
|   -O<file>      reorder diffs according to the <file>.
|   -S<string>    find filepair whose only one side contains the string.
|   --pickaxe-all
|                 show all files diff when -S is used and hit is found.
|   -a  --text    treat all files as text.

The problem is not critical, as the build process continues with wrong
version information.

mfg Martin Kögler