Do not break with long releases

If release contains many commits and/or those commits contain many
changed files, the command line could grow past OS limit.  Using stdin
avoids this.

src/include/release.sh

     #
     local rlevel=$1     # bump level (x, y or z)
     local nextver       # version after the bump
+    local cache         # cache for the message
     __relck git_present
     __relck at_relsrc
     __relck not_dirty
       || die "failed to update version in mkit.ini"
     git add mkit.ini \
       || die "failed to add mkit.ini to the index"
-    git commit -e -m "$(_vbump_gitmsg)"
+    cache=$(mktemp -t "mkit._vbump_gitmsg.XXXXXXXX")
+    _vbump_gitmsg > "$cache"
+    git commit -e -F "$cache"   # note: reading from stdin will break vim
+    rm "$cache"
 }
 
 _vbump_gitmsg() {