gh-159440: Pass --work-tree to the git commands used for the version tag (#159442)

`--git-dir` alone does not imply a work tree, so in an out-of-tree build
git checked the build directory, and every tracked file looked deleted:
`sys.version` always carried a spurious "-dirty" suffix.

Author: sendaoYan

Misc/NEWS.d/next/Build/2026-10-15-11-20-48.gh-issue-159440.uIz6ms.rst

@@ -0,0 +1,3 @@
+The version string reported by ``sys.version`` no longer carries a spurious
+``-dirty`` suffix when CPython is built out-of-tree from a clean git checkout:
+the build stamp now checks the source tree instead of the build directory.

configure

@@ -128,9 +128,9 @@ HAS_GIT=no-repository
 fi
 if test $HAS_GIT = found
 then
-    GITVERSION="git --git-dir \$(srcdir)/.git rev-parse --short HEAD"
-    GITTAG="git --git-dir \$(srcdir)/.git describe --all --always --dirty"
-    GITBRANCH="git --git-dir \$(srcdir)/.git name-rev --name-only HEAD"
+    GITVERSION="git --git-dir \$(srcdir)/.git --work-tree \$(srcdir) rev-parse --short HEAD"
+    GITTAG="git --git-dir \$(srcdir)/.git --work-tree \$(srcdir) describe --all --always --dirty"
+    GITBRANCH="git --git-dir \$(srcdir)/.git --work-tree \$(srcdir) name-rev --name-only HEAD"
 else
     GITVERSION=""
     GITTAG=""