tag-git: be smarter when determining the base tag

Whether rebasing to, merging-rebasing to, or merging an upstream tag, or
not integrating an upstream tag at all, we must figure out the best base
tag. So far, I tried several strategies:

- `--first-parent`: This works only for merging-rebases

- `--exclude='*.windows.*'`: This works if integrating an upstream tag,
  but not when a `.windows.2` version is desired

- calling `git describe` twice: first to figure out the reachable
  upstream tag that is closest to the revision, and then
  calling it a second time, this time allowing for a Git for Windows
  tag. If the two searches came up with different results, pick the
  most recent one.

  Unfortunately that did not work. Its logic relies on the date when the
  Git version the downstream Git for Windows version was tagged.

  This broke down in the case of v2.50.1, which was tagged before I
  could release v2.50.0.windows.1.

Let's just instead pick the higher version number by leveraging the
`version_compare` function I developed in
git-for-windows/build-extra@1418ee7e8e
(git-update: do not suggest downgrading from an -rc version, 2017-10-26)
and that has been subsequently been enhanced in that repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

update-scripts/tag-git.sh

@@ -28,6 +28,43 @@ git_rev="$1"
 test "refs/heads/$release_branch" = "$(git -C "$build_extra_dir" symbolic-ref HEAD)" ||
 die "Need the current branch in '$build_extra_dir' to be '$release_branch'"
 
+# This function was copied from `git-update-git-for-windows` in
+# the `mingw-w64-git-extra` package.
+version_compare () {
+        a="$1"
+        b="$2"
+
+        while true
+        do
+                test -n "$b" || { echo 1; return; }
+                test -n "$a" || { echo -1; return; }
+
+                # Get the first numbers (if any)
+                a1="$(expr "$a" : '^\([0-9]*\)')"; a="${a#$a1}"
+                b1="$(expr "$b" : '^\([0-9]*\)')"; b="${b#$b1}"
+
+                if test -z "$b1"
+                then
+                        test -z "$a1" || { echo 1; return; }
+                        a1=0
+                        b1=0
+                fi
+                test -n "$a1" || { echo -1; return; }
+                test $a1 -le $b1 || { echo 1; return; }
+                test $b1 -le $a1 || { echo -1; return; }
+
+                # Skip non-numeric prefixes
+                a1="$(expr "$a" : '^\([^0-9]\+\)')"; a="${a#$a1}"
+                b1="$(expr "$b" : '^\([^0-9]\+\)')"; b="${b#$b1}"
+
+                case "$a1,$b1" in
+                [.-]rc,[.-]rc) ;; # both are -rc versions
+                [.-]rc,*) echo -1; return;;
+                *,[.-]rc) echo 1; return;;
+                esac
+        done
+}
+
 mkdir -p "$artifacts_dir" &&
 if test -n "$snapshot_version"
 then
@@ -45,6 +82,13 @@ else
 
 	desc="$(git --git-dir "$git_git_dir" describe --match 'v[0-9]*[0-9]' --exclude='*-[0-9]*' --exclude='*.windows.*' "$git_rev")" &&
 	base_tag=${desc%%-[1-9]*} &&
+	desc2="$(git --git-dir "$git_git_dir" describe --match 'v[0-9]*[0-9]' --exclude='*-[0-9]*' "$git_rev")" &&
+	base_tag2=${desc2%%-[1-9]*} &&
+	# If there is a candidate Git tag _and_ a candidate Git for Windows tag, compare versions
+	if test "$base_tag" != "$base_tag2" && test $(version_compare "$base_tag" "$base_tag2") -lt 0
+	then
+		base_tag="$base_tag2"
+	fi &&
 	case "$base_tag" in
 	"$desc") die "Revision '$git_rev' already tagged as $base_tag";;
 	*.windows.*)