maint: make ./bootstrap abort if no .git/ directory is present.

maint: make ./bootstrap abort if no .git/ directory is present.

In response to (long thread continuing over a month boundary):
https://lists.gnu.org/archive/html/automake/2025-05/msg00013.html
https://lists.gnu.org/archive/html/automake/2025-06/msg00000.html

* bootstrap: if no .git directory is present, abort with
a long-winded error message. Nothing good comes of running
./bootstrap in a tree from a release tarball, e.g., makeinfo
suddenly becomes required, per the original report.