When a merge request is created from email, use message body
as merge request description. If message body is empty then
merge request description is still created from the source
branch commit (if there is only single commit in the merge
request).

If message body is empty and there are multiple commits in
the source branch, then merge request description is left empty.

Closes #40968

On GitLab.com, InfluxSampler#sample_objects appears to take 1.2 s or so to
iterate through 1059 objects. This had led to delays of a couple hundred
milliseconds in processing in the main thread. Remove this code since it's not
really being used.

Closes gitlab-com/infrastructure#3250

If the source import directory were different from the destination directory,
GitLab would first create a blank repository and then erroneously move the
original one into a subdirectory. Adding an import type prevents this the project
from being initialized in the first place. It was accidentally removed in
1f917cbd49.

Closes #40765

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

- handle unchanged empty lines in inline diff

+ change string concatenation to help with GC pressure.
+ fix metric producing incompatible label sets

This also allows us to simplify the naming since we can make some
fetching methods private.

Use Class.new(StandardError) instead of custom extended error class. Bring back specified_dependencies?.

Signed-off-by: NRémy Coutable <remy@rymai.me>

- Added tests for Git::Commit.shas_with_signatures

Closes gitaly#780

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

In particular, the Omnibus uploads directory is generally a symlink.