This change from microsoft#468 is causing multiple maintenance
processes to get blocked on credentials instead of only one. The change
did more harm than good.

This reverts commit 95ed7f6.

When scripts or background maintenance wish to perform HTTP(S) requests,
there is a risk that our stored credentials might be invalid. At the
moment, this causes the credential helper to ping the user and block the
process. Even if the credential helper does not ping the user, Git falls
back to the 'askpass' method, which includes a direct ping to the user
via the terminal.

Even setting the 'core.askPass' config as something like 'echo' will
causes Git to fallback to a terminal prompt. It uses
git_terminal_prompt(), which finds the terminal from the environment and
ignores whether stdin has been redirected. This can also block the
process awaiting input.

Create a new config option to prevent user interaction, favoring a
failure to a blocked process.

The chosen name, 'credential.interactive', is taken from the config
option used by Git Credential Manager to already avoid user
interactivity, so there is already one credential helper that integrates
with this option. However, older versions of Git Credential Manager also
accepted other string values, including 'auto', 'never', and 'always'.
The modern use is to use a boolean value, but we should still be
careful that some users could have these non-booleans. Further, we
should respect 'never' the same as 'false'. This is respected by the
implementation and test, but not mentioned in the documentation.

The implementation for the Git interactions takes place within
credential_getpass(). The method prototype is modified to return an
'int' instead of 'void'. This allows us to detect that no attempt was
made to fill the given credential, changing the single caller slightly.

Also, a new trace2 region is added around the interactive portion of the
credential request. This provides a way to measure the amount of time
spent in that region for commands that _are_ interactive. It also makes
a conventient way to test that the config option works with
'test_region'.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

At the moment, some background jobs are getting blocked on credentials
during the 'prefetch' task. This leads to other tasks, such as
incremental repacks, getting blocked. Further, if a user manages to fix
their credentials, then they still need to cancel the background process
before their background maintenance can continue working.

Update the background schedules for our four scheduler integrations to
include these config options via '-c' options:

 * 'credential.interactive=false' will stop Git and some credential
   helpers from prompting in the UI (assuming the '-c' parameters are
   carried through and respected by GCM).

 * 'core.askPass=true' will replace the text fallback for a username
   and password into the 'true' command, which will return a success in
   its exit code, but Git will treat the empty string returned as an
   invalid password and move on.

We can do some testing that the credentials are passed, at least in the
systemd case due to writing the service files.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

The 'scalar reconfigure' command is intended to update registered repos
with the latest settings available. However, up to now we were not
reregistering the repos with background maintenance.

In particular, this meant that the background maintenance schedule would
not be updated if there are improvements between versions.

Be sure to register repos for maintenance during the reconfigure step.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

In this commit, we added the 'credential.interactive=never' option to
unattended scalar options. This should be changed to 'false' to match
the modern use of this config option.

But also, we have a test that requires using askpass to get credentials,
but the test is in unattended mode. Fix that test to include
'credential.interactive=true' to bypass this issue.