bootstrap: improve gnulib git update logic

[Kai Torben Ohlhus]

Dear Gnulib maintainers,

The GNU Octave project successfully uses your great library for many
years.  We would like to contribute a local change the Octave project
has tested for the past half year on your bootstrap script [1,2,3] (also
attached as file to this email):

------------------------------------------------------------
diff --git a/build-aux/bootstrap b/build-aux/bootstrap
index 8f76d6962..3a87e7f7e 100755
--- a/build-aux/bootstrap
+++ b/build-aux/bootstrap
@@ -670,6 +670,11 @@ if $use_gnulib; then
         || cleanup_gnulib

       trap - 1 2 13 15
+
+    elif test -n "$GNULIB_REVISION" \
+         && ! git --git-dir="$gnulib_path"/.git cat-file \
+              commit "$GNULIB_REVISION"; then
+      git --git-dir="$gnulib_path"/.git fetch
     fi
     GNULIB_SRCDIR=$gnulib_path
     ;;
------------------------------------------------------------

The idea is as follows (with dates as fake revision IDs):

- We have an "old" gnulib clone (ID 2020-08-01) and want to build
  a newer revision (ID 2020-08-15)

- If we run in the "old" gnulib clone

     GNULIB_REVISION=2020-08-15 ./bootstrap

  we get the message

     fatal: reference is not a tree: 2020-08-15

  even though revision 2020-08-15 exists.  It is just not available
  in the "old" local clone.

To resolve it, we either have to update (fetch/pull) the gnulib clone
manually, or we developed the aforementioned patch to fetch recent
changes, if the desired revision is not fetched in the local clone.

Thank you for your time and help,

Kai


[1] https://savannah.gnu.org/bugs/?57044
[2]
https://octave.discourse.group/t/update-gnulib-ahead-of-octave-6-release-candidate/76
[3] https://hg.savannah.gnu.org/hgweb/octave/rev/717e3780805c
    (still using "pull", later changed to "fetch")

bootstrap.diff
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