filemerge: when using in-memory merge, always put backup files in temp dir
Before calling a merge tool, we create a backup of the local side of
the merge. That file can be put in the working copy or in a temporary
directory, depending on the user's config. When we're merging in
memory, we don't want to write to the actual, on-disk working copy, so
we write the file to the in-memory working copy instead. However,
since we don't support external merge tools with in-memory merge, it
makes no difference where the file is actually stored (and if we ever
do add support for external merge tools, then the file clearly can't
live in the in-memory working-copy object anyway). So, since it
doesn't matter where the file is stored, we can simplify by always
putting them in the system's temp directory.
Differential Revision: https://phab.mercurial-scm.org/D12187
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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Notes for packagers
===================
Mercurial ships a copy of the python-zstandard sources. This is used to
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