Make sure the name of the temporary file ends with "+"

To avoid regression by leaving untracked cruft that is not covered
by the .gitignore file, match the convention to generate into
$name$suffix and then move to $name where $suffix ends with "+",
which is used everywhere else in the system.

The paragraph

    Incidentally, this also fixes something else: The `+` character is
    not even a valid filename character on Windows. The only reason why Git
    for Windows did not need this is that above-mentioned POSIX emulation
    layer also plays a couple of tricks with filenames (tricks that are not
    interoperable with regular Windows programs, though), and previous
    attempts to remedy this in git/git were unsuccessful, see e.g.
    https://lore.kernel.org/git/[email protected]/

no longer applies.  It is not like this cript was the only offender.

Author: gitster

GIT-VERSION-GEN

@@ -78,6 +78,8 @@ read GIT_MAJOR_VERSION GIT_MINOR_VERSION GIT_MICRO_VERSION GIT_PATCH_LEVEL trail
 $(echo "$GIT_VERSION" 0 0 0 0 | tr '.a-zA-Z-' ' ')
 EOF
 
+OUTPUT_TMP="$OUTPUT.$$+"
+
 sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
 	-e "s|@GIT_MAJOR_VERSION@|$GIT_MAJOR_VERSION|" \
 	-e "s|@GIT_MINOR_VERSION@|$GIT_MINOR_VERSION|" \
@@ -86,11 +88,11 @@ sed -e "s|@GIT_VERSION@|$GIT_VERSION|" \
 	-e "s|@GIT_BUILT_FROM_COMMIT@|$GIT_BUILT_FROM_COMMIT|" \
 	-e "s|@GIT_USER_AGENT@|$GIT_USER_AGENT|" \
 	-e "s|@GIT_DATE@|$GIT_DATE|" \
-	"$INPUT" >"$OUTPUT".$$
+	"$INPUT" >"$OUTPUT_TMP"
 
-if ! test -f "$OUTPUT" || ! cmp "$OUTPUT".$$ "$OUTPUT" >/dev/null
+if ! test -f "$OUTPUT" || ! cmp "$OUTPUT_TMP" "$OUTPUT" >/dev/null
 then
-	mv "$OUTPUT".$$ "$OUTPUT"
+	mv "$OUTPUT_TMP" "$OUTPUT"
 else
-	rm "$OUTPUT".$$
+	rm "$OUTPUT_TMP"
 fi