Some Mercurial commands deal with writing patches. On these there is an option "--git".
Excerpt from 'hg help export':
Use the -g/--git option to generate diffs in the git extended diff format. See 'hg help diffs' for more information.
'hg help' topic about diffs
Mercurial has a special help topic about diffs (https://www.mercurial-scm.org/doc/hg.1.html#diff-formats):
> hg help diffs Diff Formats 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.
Recurring topic on mailing lists
Asking for defining the --git option as a default is a recurring topic on the mailing lists.
Matt Mackall summarized on 2009-08-16 his decisions on the topic as follows (https://www.mercurial-scm.org/pipermail/mercurial-devel/2009-August/014752.html):
Here's how this topic goes, every time for the past four years: Q: Can we make --git the default? A: No, it's non-standard. Q: How about just for mq? A: No. Q: Can we get a warning or something when a patch is going to lose information? A: Sounds great, send a patch.
Support was added in mq for versions 1.4.3 and later. See mq.py for more details about configuration.
Relevant issues in bug tracker
https://bz.mercurial-scm.org/show_bug.cgi?id=767 - "hg qref loses data or metadata without warning"