Preserve information on use of git-external-diff

Now that the Working with git wiki page no longer suggests producing
context diffs, we should preserve the information on how to use
git-external-diff for those people who want to view context format
diffs. The most obvious place is in the script itself, so that's what's
done here.

src/tools/git-external-diff

@@ -7,6 +7,41 @@
 # path old-file old-hash old-mode new-file new-hash new-mode
 # 'path' is the git-tree-relative path of the file being diff'ed
 
+=comment
+
+This info is copied from the old wiki page on Working with git:
+
+Context diffs with Git
+
+Copy git-external-diff into libexec/git-core/ of your git installation
+and configure git to use that wrapper with:
+
+    git config [--global] diff.external git-external-diff
+
+--global makes the configuration global for your user - otherwise it is
+just configured for the current repository.
+
+For every command which displays diffs in some way you can use the
+parameter "--[no-]-ext-diff" to enable respectively disable using the
+external diff command.
+
+For the git diff command --ext-diff is enabled by default - for any
+other command like git log -p or git format-patch it is not!
+
+This method should work on all platforms supported by git.
+
+If you do not want to configure the external wrapper permanently or you
+want to overwrite it you can also invoke git like:
+
+    export GIT_EXTERNAL_DIFF=git-external-diff
+    git diff --[no-]ext-diff
+
+Alternatively, configure a git alias in ~/.gitconfig or .git/config:
+
+    [alias]
+        cdiff = !GIT_EXTERNAL_DIFF=git-context-diff git diff
+=cut
+
 old_hash="$3"
 new_hash=$(git hash-object "$5")