build: make dist from a git worktree omits VERSION and the generated ChangeLog #4867

Summary

make dist in a linked worktree (git worktree add) produces a tarball without VERSION, and its ChangeLog is
the tracked one-line placeholder instead of the git log. A normal clone of the same commit produces both.

Cause

gen-VERSION and gen-ChangeLog in the top-level Makefile.am run only when test -d $(top_srcdir)/.git
($(srcdir) for the ChangeLog) is true. 4a6506c (#1856) added that guard so that make dist works without git
(#1855). In a linked worktree .git is a file (gitdir: <path>), so the test is false and both targets are
skipped, although git works there.

Impact

build-aux/pkg-version takes the version from VERSION, else from git describe, else from the current date. The
tarball's own configure is fine, because the version was baked in when autogen.sh ran. Once the tarball is
regenerated with ./autogen.sh or autoreconf, as most distributions do, there is neither a VERSION file nor
git, and the version silently becomes the date:

$ ./configure --version | head -1          # as shipped
glusterfs configure 12dev
$ ./autogen.sh >/dev/null && ./configure --version | head -1
glusterfs configure 2026.10.21

Release branches are commonly checked out as worktrees, so this can reach a real release tarball.

Proposal

Run both targets when .git exists and git works there (test -e plus git -C ... rev-parse --git-dir), PR #4868.

• A tree without git, or with a .git entry git cannot use (a worktree whose main repository moved), still skips
  both targets, which is what Makefile: failed to compile without git repository #1855 needs.
• A clone on a machine without git, where make dist fails today, skips them too.