doc: add caveat about round-tripping format-patch

git-format-patch(1) and git-am(1) deal with formatting commits as
patches and applying them, respectively. Naturally they use a few
delimiters to mark where the commit message ends. This can lead to
surprising behavior when these delimiters are used in the commit
message itself.

git-format-patch(1) will accept any commit message and not warn or error
about these delimiters being used.[1]

Especially problematic is the presence of unindented diffs in the commit
message; the patch machinery will naturally (since the commit message
has ended) try to apply that diff and everything after it.[2]

It is unclear whether any commands in this chain will learn to warn
about this. One concern could be that users have learned to rely on
the three-dash line rule to conveniently add extra-commit message
information in the commit message, knowing that git-am(1) will
ignore it.[4]

All of this is covered already, technically. However, we should spell
out the implications.

† 1: There is also git-commit(1) to consider. However, making that
     command warn or error out over such delimiters would be disruptive
     to all Git users who never use email in their workflow.
† 2: Recently patch(1) caused this issue for a project, but it was noted
     that git-am(1) has the same behavior[3]
† 3: https://github.com/i3/i3/pull/6564#issuecomment-3858381425
† 4: https://lore.kernel.org/git/xmqqldh4b5y2.fsf@gitster.g/
     https://lore.kernel.org/git/V3_format-patch_caveats.354@msgid.xyz/

Reported-by: Matthias Beyer <mail@beyermatthias.de>
Reported-by: Christoph Anton Mitterer <calestyo@scientia.org>
Reported-by: Matheus Tavares <matheus.tavb@gmail.com>
Reported-by: Chris Packham <judge.packham@gmail.com>
Helped-by: Jakob Haufe <sur5r@sur5r.net>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/format-patch-caveats.adoc

@@ -0,0 +1,33 @@
+The output from linkgit:git-format-patch[1] can lead to a different
+commit message when applied with linkgit:git-am[1]. The patch that is
+applied may also be different from the one that was generated, or patch
+application may fail outright.
+ifdef::git-am[]
+See the <<discussion,DISCUSSION>> section above for the syntactic rules.
+endif::git-am[]
+
+ifndef::git-am[]
+include::format-patch-end-of-commit-message.adoc[]
+endif::git-am[]
+
+Note that this is especially problematic for unindented diffs that occur
+in the commit message; the diff in the commit message might get applied
+along with the patch section, or the patch application machinery might
+trip up because the patch target doesn't apply. This could for example
+be caused by a diff in a Markdown code block.
+
+The solution for this is to indent the diff or other text that could
+cause problems.
+
+This loss of fidelity might be simple to notice if you are applying
+patches directly from a mailbox. However, changes originating from Git
+could be applied in bulk, in which case this would be much harder to
+notice. This could for example be a Linux distribution which uses patch
+files to apply changes on top of the commits from the upstream
+repositories. This goes to show that this behavior does not only impact
+email workflows.
+
+Given these limitations, one might be tempted to use a general-purpose
+utility like patch(1) instead. However, patch(1) will not only look for
+unindented diffs (like linkgit:git-am[1]) but will try to apply indented
+diffs as well.

Documentation/format-patch-end-of-commit-message.adoc

@@ -0,0 +1,8 @@
+Any line that is of the form:
+
+* three-dashes and end-of-line, or
+* a line that begins with "diff -", or
+* a line that begins with "Index: "
+
+is taken as the beginning of a patch, and the commit log message
+is terminated before the first occurrence of such a line.

Documentation/git-am.adoc

@@ -233,6 +233,7 @@ applying.
 	create an empty commit with the contents of the e-mail message
 	as its log message.
 
+[[discussion]]
 DISCUSSION
 ----------
 
@@ -252,14 +253,11 @@ where the patch begins.  Excess whitespace at the end of each
 line is automatically stripped.
 
 The patch is expected to be inline, directly following the
-message.  Any line that is of the form:
+message.
+include::format-patch-end-of-commit-message.adoc[]
 
-* three-dashes and end-of-line, or
-* a line that begins with "diff -", or
-* a line that begins with "Index: "
-
-is taken as the beginning of a patch, and the commit log message
-is terminated before the first occurrence of such a line.
+This means that the contents of the commit message can inadvertently
+interrupt the processing (see the <<caveats,CAVEATS>> section below).
 
 When initially invoking `git am`, you give it the names of the mailboxes
 to process.  Upon seeing the first patch that does not apply, it
@@ -283,6 +281,13 @@ commits, like running 'git am' on the wrong branch or an error in the
 commits that is more easily fixed by changing the mailbox (e.g.
 errors in the "From:" lines).
 
+[[caveats]]
+CAVEATS
+-------
+
+:git-am: 1
+include::format-patch-caveats.adoc[]
+
 HOOKS
 -----
 This command can run `applypatch-msg`, `pre-applypatch`,

Documentation/git-format-patch.adoc

@@ -798,6 +798,10 @@ if they are part of the requested range. A simple "patch" does not
 include enough information for the receiving end to reproduce the same
 merge commit.
 
+=== PATCH APPLICATION
+
+include::format-patch-caveats.adoc[]
+
 SEE ALSO
 --------
 linkgit:git-am[1], linkgit:git-send-email[1]

Documentation/git-send-email.adoc

@@ -692,6 +692,11 @@ Links of a few such community maintained helpers are:
 	- https://github.com/AdityaGarg8/git-credential-email[git-msgraph]
 	  (cross platform client that can send emails using the Microsoft Graph API)
 
+CAVEATS
+-------
+
+include::format-patch-caveats.adoc[]
+
 SEE ALSO
 --------
 linkgit:git-format-patch[1], linkgit:git-imap-send[1], mbox(5)