rerere affects handling of git-stash-pop merge conflicts

[Phil Hord <hordp@cisco.com>]

I was investigating a git-stash-pop anomaly when I ran across this one.
I think this is residual breakage from [1] bb0a484 (mergetool: Skip
autoresolved paths, 2010-08-17).  A similar problem aimed at 'git merge'
was ostensibly fixed by [2] 2f59c9 (mergetool: don't skip modify/remove
conflicts, 2011-02-16).

[1] http://thread.gmane.org/gmane.comp.version-control.git/153420
[2] http://thread.gmane.org/gmane.comp.version-control.git/164622


Summary:
After a 'git stash pop' with conflicts, 'git mergetool' fails to notice
the conflicted files if 'rerere.enabled=true'.  git mergetool
erroneously reports 'no files need merging'.

After a 'git merge' with conflicts, 'git mergetool' seems to work ok in
either case.

Here's my test script:

------8<------ /tmp/stash.sh
#!/bin/bash

#enable/disable rerere for this test
git config --global rerere.enabled ${1:false}

# run the test
rm -rf foo
mkdir foo && cd foo && git init &&
echo A>file && git add file && git commit --quiet -m "A" &&
echo B>file && git stash &&
echo C>file && git add file && git commit --quiet -m "C" &&
git stash pop

git mergetool
------8<------ /tmp/stash.sh

git --version
    git version 1.7.6.178.g55272

# With rerere.enabled=false, it works ok (mergetool offers to
# merge the conflicting file):
./stash.sh false

    Initialized empty Git repository in /tmp/foo/.git/
    Saved working directory and index state WIP on master: ac67a86 A
    HEAD is now at ac67a86 A
    Auto-merging file
    CONFLICT (content): Merge conflict in file
    merge tool candidates: meld opendiff kdiff3 tkdiff xxdiff
    tortoisemerge gvimdiff diffuse ecmerge p4merge araxis bc3 vimdiff
    emerge
    Merging:
    file

    Normal merge conflict for 'file':
      {local}: modified
      {remote}: modified
    Hit return to start merge resolution tool (xxdiff): ^C

# But with rerere.enabled=true, mergetool thinks there is no conflict:
./stash.sh true

    Initialized empty Git repository in /tmp/foo/.git/
    Saved working directory and index state WIP on master: d40e77b A
    HEAD is now at d40e77b A
    Auto-merging file
    CONFLICT (content): Merge conflict in file
    merge tool candidates: meld opendiff kdiff3 tkdiff xxdiff
    tortoisemerge gvimdiff diffuse ecmerge p4merge araxis bc3 vimdiff
    emerge
    No files need merging

Phil