Add ABCL Git conversion post.

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+;;;;;
+title: Setting up an ABCL mirror from SVN to Git
+date: 2016-08-03 12:03:02
+format: md
+tags: git
+;;;;;
+
+## Copies of copies
+
+As it so happens the ABCL main repository is held in Subversion, but since I'm
+more productive with Git I usually use a conversion to Git via the `git-svn`
+program.  I've based my own copy off of [slryus'
+one](https://github.com/slyrus/abcl), however I had to fetch and update my own
+local copy since neither of us had done any fetching in quite some time.  I'm
+writing down some notes below to make sure I (or others) can repeat the process
+in the future.
+
+First we need a copy of an existing Git conversion.
+
+    git clone https://github.com/Ferada/abcl.git abcl-git
+    # or in case you have SSH access
+    git clone git@github.com:Ferada/abcl.git abcl-git
+
+The `master` branch should be a direct mirror of the SVN repository, i.e. only
+have commits with `git-svn` annotations, like
+`git-svn-id: http://abcl.org/svn/trunk/abcl@14851 1c010e3e-...`.
+
+Next we want to initialise the SVN remote and fetch the new commits.
+
+    git svn init --prefix=abcl/ http://abcl.org/svn/trunk/abcl
+    git svn fetch -r 14791:14851
+
+Note that the first revision in the `fetch` command is the last one in the Git
+`master` branch and the other one is the current `HEAD` of the SVN repository.
+
+This process will take just a few moments, however while all the new commits
+will be based off of the `master` branch in Git, the first commit will be a
+duplicate and have more changes than the existing commit in `Git`.
+
+    2015-08-31 20:55 mevenson           │ │ o │ │ │ ansi-test: reference new git repository
+    2015-07-01 04:16 mevenson           o─┴─│─┴─┴─┘ abcl-asdf: fix usage with local repository   <<<<
+    2015-07-01 04:16 mevenson           │ I─┘ abcl-asdf: fix usage with local repository         <<<<
+    2015-06-30 18:42 mevenson           o abcl-asdf: correct metadata
+
+The newly created first commit (14791 here) has lots of changes.
+
+    .dir-locals.el
+    CHANGES
+    COPYING
+    MANUAL
+    README
+    abcl.asd
+    abcl.bat.in
+    ...
+
+While the second, pre-existing one, has way less (and is just showing the same
+changes that are in the SVN commit).
+
+    contrib/abcl-asdf/abcl-asdf.asd
+    contrib/abcl-asdf/abcl-asdf.lisp
+    contrib/abcl-asdf/maven-embedder.lisp
+
+To correct that and continue with a linear history I'm using the interactive
+rebase command.
+
+    git co -b test
+    git reset --hard abcl/git-svn
+    git rebase -i <commit _before_ the duplicated one>
+
+Copy the hash of the correct, smaller commit and replace the `pick ...` line
+(at the top) that contains the duplicated commit with the one from the
+cloned-from Git repository, then exit the editor.  Once the rebase is done we
+just need to update the ref for `git-svn`.
+
+    git update-ref refs/remotes/abcl/git-svn test
+
+Note the syntax.  If done correctly it will be updated in e.g. `tig`, if not
+there'll be a *new* entry showing up in `git show-ref`.
+
+Lastly we want to rebase `master` to get the new commits.
+
+    git co master
+    git co -D test
+    git rebase abcl/git-svn master
+
+Which will look somewhat like this:
+
+    2016-06-13 08:06 mevenson           o [master] {abcl/git-svn} {github/master} doc: note changes for abcl-1.3.4
+    2016-05-16 23:05 mevenson           o Update to asdf-3.1.7
+    2016-05-16 21:43 mevenson           o Update to jna-4.2.2
+
+And push the updates.
+
+    git push origin master