[RFC PATCH 2/2] git-p4: support loading changelist descriptions from files

Our Perforce server experienced some kind of database corruption a few years ago.
While the file data and revision history are mostly intact,
some metadata for several changesets got lost.
For example, inspecting certain changelists produces errors.
"""
$ p4 describe -s 12345
Date 2019/02/26 16:46:17:
Operation: user-describe
Operation 'user-describe' failed.
Change 12345 description missing!
"""

While some metadata (like changeset descriptions) is obviously lost,
most of it can be reconstructed via other commands:
 * `p4 changes -l -t //...@12345,12345` --
   to obtain date+time, author, beginning of changeset description;
 * `p4 files -a //...@12345,12345` --
   to obtain file revisions, file types, file actions;
 * `p4 diff2 -u //...@12344 //...@12345` --
   to get a unified diff of text files in a changeset;
 * `p4 print -o binary.blob@12345 //depot/binary.blob@12345` --
   to get a revision of a binary file.

It might be possible to teach git-p4 to fallback to other methods if `p4 describe` fails, but it's probably too special-cased (really depends on kind and scale of DB corruption), so some manual intervention is perhaps acceptable.

So, with some manual work, it's possible to reconstruct `p4 -G describe ...` output manually. In our case, once git-p4 passes `p4 describe` stage, it can proceed further just fine. Thus, it's tempting to feed resurrected metadata to git-p4 when a normal `p4 describe` would fail.

This functionality may be useful to cache changelist information, or to make some changes to changelist info before feeding it to git-p4.

A new config parameter is introduced to tell git-p4 to load certain changelist descriptions from files instead of from a server. For simplicity, it's one pickled file per changelist. ``` git config --add git-p4.damagedChangelists 12345.pickled git config --add git-p4.damagedChangelists 12346.pickled ```

The following trivial script may be used to produce pickled `p4 -G describe`-compatible output.
"""
 #!/usr/bin/python2

 import pickle
 import time

 # recovered commits of interest
 changes = [
     {
         'change':     '12345',
         'status':     'submitted',
         'code':       'stat',
         'user':       'username1',
         'time':       str(int(time.mktime(time.strptime('2019/02/28 16:00:30', '%Y/%m/%d %H:%M:%S')))),
         'client':     'username1_hostname1',
         'desc':       'A bug is fixed.\nDetails are below:<lost>\n',
         'depotFile0': '//depot/branch1/foo.sh',
         'action0':    'edit',
         'rev0':       '28',
         'type0':      'xtext',
         'depotFile1': '//depot/branch1/bar.py',
         'action1':    'edit',
         'rev1':       '43',
         'type1':      'text',
         'depotFile2': '//depot/branch1/baz.doc',
         'action2':    'edit',
         'rev2':       '8',
         'type2':      'binary',
         'depotFile3': '//depot/branch1/qqq.c',
         'action3':    'edit',
         'rev3':       '6',
         'type3':      'ktext',
     },
 ]

 for change in changes:
     pickle.dump(change, open('{0}.pickled'.format(change['change']), 'wb'))
"""

Signed-off-by: Andrey Mazo <amazo@checkvideo.com>
---

Notes:
    Documentation changes and tests are obviously missing,
    but I hoped to get some feedback on the idea overall
    before working on those.

 git-p4.py | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)