mercurial/helptext/diffs.txt

debugmergestate: make templated

Our IntelliJ team wants to be able to read the merge state in order to
help the user resolve merge conflicts. They had so far been reading
file contents from p1() and p2() and their merge base. That is not
ideal for several reasons (merge base is not necessarily the "graft
base", renames are not handled, commands like `hg update -m` is not
handled). It will get especially bad as of my D7827. This patch makes
the output s a templated. I haven't bothered to make it complete
(e.g. merge driver states are not handled), but it's probably good
enough as a start.

I've done a web search for "debugmergestate" and I can't find any
indication that any tools currently rely on its output. If it turns
out that we get bug reports for it once this is released, I won't
object to backing this patch out on the stable branch (and then
perhaps replace it by a separate command, or put it behind a new
flag).

The changes in test-backout.t are interesting, in particular this:

```
  -    other path: foo (node not stored in v1 format)
  +    other path:  (node foo)
```

I wonder if that means that we actually read v1 format
incorrectly. That seems to be an old format that was switched away
from in 2014, so it doesn't matter now anyway.

Differential Revision: https://phab.mercurial-scm.org/D8120

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 configuration file. You do not need to set this option
when importing diffs in this format or using them in the mq extension.