tools: a simple git.mergetool for working with buildout.hash.cfg

Rewriting commit history with `git rebase -i` always caused some
conflicts on md5sum of modified files. `update-hash` made all this
easier, because we could just revert buildout.hash.cfg, re-run
`update-hash` and commit the changes.

This tool makes this scenario a bit more user friendly by automating the
steps by running as a git mergetool.

An interactive rebase session can be initiated by running this command
from software/*/ directory:

`git rebase -i --exec "$(pwd)/../../update-hash $(pwd)/buildout.hash.cfg" origin/master`

and if conflict occur, run:

`git mergetool --tool update-hash-mergetool && EDITOR=cat git rebase --continue`

This way, rebase session can be almost non interactive (still have to
run the above mergetool command) when conflicts are only on md5sum
updates.

/reviewed-on !273

update-hash-mergetool

+#!/bin/bash -e
+# simple mergetool for buildout.hash.cfg - just update hash
+
+# suggested installation:
+#  git config merge.tool update-hash-mergetool
+#  git config mergetool.update-hash-mergetool.cmd './update-hash-mergetool "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'
+#  git config mergetool.update-hash-mergetool.trustExitCode true
+
+
+BASE=$1
+LOCAL=$2
+REMOTE=$3
+MERGED=$4
+
+
+if echo "$MERGED" | grep -q "buildout.hash.cfg"; then
+    cp $REMOTE $MERGED
+    exec ./update-hash $MERGED
+else
+    echo "this mergetool only supports buildout.hash.cfg" 1>&2
+    exit 1
+fi