typo here "do" -> "to"

stylistic matter: the src/dest variables are used only once (hence no speed up in using them), and naming them doesn't add anything to the readability of the code. It is minor point, in cases like this one putting the regexps directly in the sub() arguments would be preferrable

extra empty line here? or is it intended?

same here: extra empty line?

same for the 3 empty lines here: are they intended? if so (for readability?), leave them around, otherwise please remove them

several constants here seem to be reused in multiple tests, at least:

• the commit id
• the directory id
• the committer name, fullname, and email

please define them either as module-level constants, or as class-level attributes of the test module, and reference them from the various tests where they are used (using self.* or not, depending on the choice)

details about this artifact look like other candidates for defining constants/attributes, and referencing them from multiple tests

here's another one: this revision, or at least many metadata about it, are reused in multiple tests. Please factorize this out, … and DRY (Don't Repeat Yourself) :-)

other stuff to be factored out

Sure. Just note it down as low priority task, maybe?

OK, almost there! The constants you added are great and reduce duplication a lot.
… but now what is duplicated is the above records, with all its 7 fields, which appears in several places :-)

I get there are two versions of it: one with _BIN/_RAW, the other with strings, but they are just 2 which is better than 10.
So please add two additional constants, e.g.: REVISION_RAW, and REVISION, defined using the other more fine grained constants, and in places like this just go with:

mock_backend.revision_get = MagicMock(return_value=REVISION_RAW)

this will become

self.assertEqual(actual_revision, REVISION)

this OTOH it is totally fine as is, as you need a finer grained version of the macro-constant

This could remain as is, as you need to do something different for this case.
But it'd be much nicer as something like:

stub_rev = REVISION_RAW
stub_rev['id'] = ...
stub_rev['message'] = ...

which has the added benefit of making clear what is different in the test data of this test case from other ones.

(Note: the above has potentially variable aliasing issues though, so you should either use a copy constructor, or make sure the constants are in a place that get refreshed for each test case.)

etc.