Feature branch support in Postgres importer

**Current behavior**

```
./poetryw run hunter regressions window1min_time_rocks_on_close --branch 3.5
Traceback (most recent call last):
File "", line 1, in
File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 516, in main
script_main(conf)
File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 666, in script_main
regressions = hunter.regressions(
File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 286, in regressions
target_series = importer.fetch_data(test, selector).analyze(options=options)
File "/Users/asorokoumov/Projects/hunter/hunter/importer.py", line 463, in fetch_data
raise ValueError("Postgres tests don't support branching yet")
ValueError: Postgres tests don't support branching yet
```

**Expected behavior**

if there are no regressions:

```
INFO: Computing change points for test window1min_time_rocks_on_close...
window1min_time_rocks_on_close: OK
No regressions found!
```

If there are regressions:

```
INFO: Computing change points for test window1min_time_rocks_on_close...
window1min_time_rocks_on_close:
process_cumulative_rate_stderr: 506. --> 940. ( +85.5%)
process_cumulative_rate_diff: 2.88e+03 --> 5.11e+03 ( +77.5%)
Regressions in 1 test found
```

Start with hunter/importer.py, especially fetch_data, where the Postgres branching error is raised, and trace the regressions command from hunter/main.py. Run the shown command with --branch 3.5; done means it completes for Postgres tests and reports either no regressions or the regression summary instead of raising the branching error.

Written by the indexing model from the issue text.