Import branch 'git-p4' of git://repo.or.cz/fast-export

Simon has asked that the git.git project include the git-p4 project
as at least a contrib/fast-import within git.git.  I think it makes
a lot of sense, as git-p4 nicely complements the only other in-tree
fast-import user: import-tars.perl.

git-p4 is offered under the MIT license by its authors.

contrib/fast-import/git-p4.bat

@@ -0,0 +1 @@
+@python "%~d0%~p0git-p4" %*

contrib/fast-import/git-p4.txt

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+git-p4 - Perforce <-> Git converter using git-fast-import
+
+Usage
+=====
+
+git-p4 supports two main modes: Importing from Perforce to a Git repository is
+done using "git-p4 sync" or "git-p4 rebase". Submitting changes from Git back
+to Perforce is done using "git-p4 submit".
+
+Importing
+=========
+
+You can simply start with
+
+  git-p4 clone //depot/path/project
+
+or
+
+  git-p4 clone //depot/path/project myproject
+
+This will create an empty git repository in a subdirectory called "project" (or
+"myproject" with the second command), import the head revision from the
+specified perforce path into a git "p4" branch (remotes/p4 actually), create a
+master branch off it and check it out. If you want the entire history (not just
+the head revision) then you can simply append a "@all" to the depot path:
+
+  git-p4 clone //depot/project/main@all myproject
+
+
+
+If you want more control you can also use the git-p4 sync command directly:
+
+  mkdir repo-git
+  cd repo-git
+  git init
+  git-p4 sync //path/in/your/perforce/depot
+
+This will import the current head revision of the specified depot path into a
+"remotes/p4/master" branch of your git repository. You can use the
+--branch=mybranch option to use a different branch.
+
+If you want to import the entire history of a given depot path just use
+
+  git-p4 sync //path/in/depot@all
+
+To achieve optimal compression you may want to run 'git repack -a -d -f' after
+a big import. This may take a while.
+
+Support for Perforce integrations is still work in progress. Don't bother
+trying it unless you want to hack on it :)
+
+Incremental Imports
+===================
+
+After an initial import you can easily synchronize your git repository with
+newer changes from the Perforce depot by just calling
+
+  git-p4 sync
+
+in your git repository. By default the "remotes/p4/master" branch is updated.
+
+It is recommended to run 'git repack -a -d -f' from time to time when using
+incremental imports to optimally combine the individual git packs that each
+incremental import creates through the use of git-fast-import.
+
+
+A useful setup may be that you have a periodically updated git repository
+somewhere that contains a complete import of a Perforce project. That git
+repository can be used to clone the working repository from and one would
+import from Perforce directly after cloning using git-p4. If the connection to
+the Perforce server is slow and the working repository hasn't been synced for a
+while it may be desirable to fetch changes from the origin git repository using
+the efficient git protocol. git-p4 supports this setup by calling "git fetch origin"
+by default if there is an origin branch. You can disable this using
+
+  git config git-p4.syncFromOrigin false
+
+Updating
+========
+
+A common working pattern is to fetch the latest changes from the Perforce depot
+and merge them with local uncommitted changes. The recommended way is to use
+git's rebase mechanism to preserve linear history. git-p4 provides a convenient
+
+  git-p4 rebase
+
+command that calls git-p4 sync followed by git rebase to rebase the current
+working branch.
+
+Submitting
+==========
+
+git-p4 has support for submitting changes from a git repository back to the
+Perforce depot. This requires a Perforce checkout separate to your git
+repository. To submit all changes that are in the current git branch but not in
+the "p4" branch (or "origin" if "p4" doesn't exist) simply call
+
+    git-p4 submit
+
+in your git repository. If you want to submit changes in a specific branch that
+is not your current git branch you can also pass that as an argument:
+
+    git-p4 submit mytopicbranch
+
+You can override the reference branch with the --origin=mysourcebranch option.
+
+If a submit fails you may have to "p4 resolve" and submit manually. You can
+continue importing the remaining changes with
+
+  git-p4 submit --continue
+
+After submitting you should sync your perforce import branch ("p4" or "origin")
+from Perforce using git-p4's sync command.
+
+If you have changes in your working directory that you haven't committed into
+git yet but that you want to commit to Perforce directly ("quick fixes") then
+you do not have to go through the intermediate step of creating a git commit
+first but you can just call
+
+  git-p4 submit --direct
+
+
+Example
+=======
+
+# Clone a repository
+  git-p4 clone //depot/path/project
+# Enter the newly cloned directory
+  cd project
+# Do some work...
+  vi foo.h
+# ... and commit locally to gi
+  git commit foo.h
+# In the meantime somebody submitted changes to the Perforce depot. Rebase your latest
+# changes against the latest changes in Perforce:
+  git-p4 rebase
+# Submit your locally committed changes back to Perforce
+  git-p4 submit
+# ... and synchronize with Perforce
+  git-p4 rebase
+
+
+Implementation Details...
+=========================
+
+* Changesets from Perforce are imported using git fast-import.
+* The import does not require anything from the Perforce client view as it just uses
+  "p4 print //depot/path/file#revision" to get the actual file contents.
+* Every imported changeset has a special [git-p4...] line at the
+  end of the log message that gives information about the corresponding
+  Perforce change number and is also used by git-p4 itself to find out
+  where to continue importing when doing incremental imports.
+  Basically when syncing it extracts the perforce change number of the
+  latest commit in the "p4" branch and uses "p4 changes //depot/path/...@changenum,#head"
+  to find out which changes need to be imported.
+* git-p4 submit uses "git rev-list" to pick the commits between the "p4" branch
+  and the current branch.
+  The commits themselves are applied using git diff/format-patch ... | git apply
+