help/diffs.txt

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+++ b/help/diffs.txt	Sun Oct 04 09:59:13 2009 +0200
@@ -0,0 +1,31 @@
+    Mercurial's default format for showing changes between two
+    versions of a file is compatible with the unified format of GNU
+    diff, which can be used by GNU patch and many other standard
+    tools.
+
+    While this standard format is often enough, it does not encode the
+    following information:
+
+    - executable status and other permission bits
+    - copy or rename information
+    - changes in binary files
+    - creation or deletion of empty files
+
+    Mercurial also supports the extended diff format from the git VCS
+    which addresses these limitations. The git diff format is not
+    produced by default because a few widespread tools still do not
+    understand this format.
+
+    This means that when generating diffs from a Mercurial repository
+    (e.g. with "hg export"), you should be careful about things like
+    file copies and renames or other things mentioned above, because
+    when applying a standard diff to a different repository, this
+    extra information is lost. Mercurial's internal operations (like
+    push and pull) are not affected by this, because they use an
+    internal binary format for communicating changes.
+
+    To make Mercurial produce the git extended diff format, use the
+    --git option available for many commands, or set 'git = True' in
+    the [diff] section of your hgrc. You do not need to set this
+    option when importing diffs in this format or using them in the mq
+    extension.