pacman-helper: inline a vercmp Bash function into the patched repo-add

When `pacman-helper.sh quick_add` is run from the `pacman-packages`
action of `git-for-windows/git-for-windows-automation`, the SDK that
the action sets up is the `build-installers` flavor, which
intentionally ships only a small subset of `/usr/bin`. `vercmp.exe`
is not in that subset, so every `repo-add` invocation prints two lines
of noise of the form:

    D:/a/_temp/build-extra/repo-add: line 254: vercmp: command not found
    D:/a/_temp/build-extra/repo-add: line 254: ((: > 0 : arithmetic syntax error: operand expected (error token is "> 0 ")

(see e.g.
https://github.com/git-for-windows/git-for-windows-automation/actions/runs/27414389819/job/81026080873,
which produced 64 such pairs before the unrelated `db3` leak bit it).
The errors themselves are benign because we invoke `repo-add` without
`--prevent-downgrade`, so the suppressed "newer version already
present" warning has no functional consequence, but I would rather
not pollute release logs with them.

Adding `vercmp.exe` (or any of the other plausible interpreters that
came up while exploring this -- `awk.exe`, `perl`, `expr.exe`) to
`build-installers` would defeat the point of that minimal SDK.
Instead, extend the existing sed-based patching of `/usr/bin/repo-add`
to also inject a small Bash function called `vercmp`. The patched
copy is what `pacman-helper.sh` invokes, and `repo-add` is itself a
Bash script, so the injected function is in scope at the call site
with no PATH manipulation, no `export -f` gymnastics across process
boundaries, and `pacman-helper.sh` itself stays POSIX `sh`.

The algorithm splits each version string on runs of non-alphanumeric
characters and walks the resulting arrays segment by segment: two
numeric segments compare numerically, a numeric vs. alphanumeric
segment lets the numeric side win (matching pacman's behavior of
treating pre-release suffixes as older), and otherwise it falls back
to lexical comparison. When one string runs out of segments first,
the longer string wins unless its extra segment starts with a letter,
in which case it is treated as a pre-release. This is essentially
the same shape as the prior-art `version_compare` in
`git-extra/git-update-git-for-windows`, and it gets the cases that
the `pacman-packages` action ever encounters right; in particular it
places `2.55.0.rc0.windows.1-1` below `2.55.0.1-1`, which matters
because the RC needs to be considered older than the release. All
nine test cases from `git-update-git-for-windows --test-version-compare`
pass, and I have also verified the failing-run strings and the
strict-equality case (which the prior-art function gets wrong but
which `vercmp`'s contract requires to be `0`).

Within a mixed-alphanumeric segment such as `rc10` vs. `rc2` the
lexical fallback gets the order wrong, but Git for Windows has never
published an RC past single digits, so a full rpm-style
sub-tokenization stays out of scope. `repo-remove` next door is
patched the same way but does not call `vercmp`, so its patching
block is intentionally left alone.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

pacman-helper.sh

@@ -79,8 +79,48 @@ call_gpg () {
 repo_add () {
 	if test ! -s "$this_script_dir/repo-add"
 	then
-		# Make sure that GPGKEY is used unquoted
-		 sed 's/"\(\${\?GPGKEY}\?\)"/\1/g' </usr/bin/repo-add >"$this_script_dir/repo-add"
+		# The `build-installers` SDK that is used by the
+		# `pacman-packages` action of
+		# `git-for-windows/git-for-windows-automation` does not ship
+		# `vercmp.exe`. Inject a small Bash function of the same name
+		# (`repo-add` is a Bash script, so it picks it up at the call
+		# site without any PATH manipulation), and make sure that
+		# `GPGKEY` is used unquoted.
+		{
+			sed '1q' </usr/bin/repo-add &&
+			cat <<\VERCMP_EOF &&
+vercmp () {
+    [[ "$1" = "$2" ]] && { echo 0; return; }
+    local -a sa=( ${1//[^0-9A-Za-z]/ } )
+    local -a sb=( ${2//[^0-9A-Za-z]/ } )
+    local n=${#sa[@]}
+    (( ${#sb[@]} < n )) && n=${#sb[@]}
+    local i
+    for (( i = 0; i < n; i++ )); do
+        [[ "${sa[i]}" = "${sb[i]}" ]] && continue
+        if [[ "${sa[i]}" =~ ^[0-9]+$ && "${sb[i]}" =~ ^[0-9]+$ ]]; then
+            (( 10#${sa[i]} > 10#${sb[i]} )) && echo 1 || echo -1
+            return
+        fi
+        [[ "${sa[i]}" =~ ^[0-9]+$ ]] && { echo  1; return; }
+        [[ "${sb[i]}" =~ ^[0-9]+$ ]] && { echo -1; return; }
+        [[ "${sa[i]}" > "${sb[i]}" ]] && echo 1 || echo -1
+        return
+    done
+    (( ${#sa[@]} == ${#sb[@]} )) && { echo 0; return; }
+    local extra sign
+    if (( ${#sa[@]} > ${#sb[@]} )); then
+        extra=${sa[n]}
+        sign=1
+    else
+        extra=${sb[n]}
+        sign=-1
+    fi
+    [[ "$extra" =~ ^[A-Za-z] ]] && echo $(( -sign )) || echo $sign
+}
+VERCMP_EOF
+			sed '1d; s/"\(\${\?GPGKEY}\?\)"/\1/g' </usr/bin/repo-add
+		} >"$this_script_dir/repo-add"
 	fi &&
 	"$this_script_dir/repo-add" "$@"
 }