maintenance: use Windows scheduled tasks

Git's background maintenance uses cron by default, but this is not
available on Windows. Instead, integrate with Task Scheduler.

Tasks can be scheduled using the 'schtasks' command. There are several
command-line options that can allow for some advanced scheduling, but
unfortunately these seem to all require authenticating using a password.

Instead, use the "/xml" option to pass an XML file that contains the
configuration for the necessary schedule. These XML files are based on
some that I exported after constructing a schedule in the Task Scheduler
GUI. These options only run background maintenance when the user is
logged in, and more fields are populated with the current username and
SID at run-time by 'schtasks'.

Since the GIT_TEST_MAINT_SCHEDULER environment variable allows us to
specify 'schtasks' as the scheduler, we can test the Windows-specific
logic on other platforms. Thus, add a check that the XML file written
by Git is valid when xmllint exists on the system.

Since we use a temporary file for the XML files sent to 'schtasks', we
prefix the random characters with the frequency so it is easier to
examine the proper file during tests. Instead of an exact match on the
'args' file, we 'grep' for the arguments other than the filename.

There is a deficiency in the current design. Windows has two kinds of
applications: GUI applications that start by "winmain()" and console
applications that start by "main()". Console applications are attached
to a new Console window if they are not already associated with a GUI
application. This means that every hour the scheudled task launches a
command window for the scheduled tasks. Not only is this visually
obtrusive, but it also takes focus from whatever else the user is
doing!

A simple fix would be to insert a GUI application that acts as a shim
between the scheduled task and Git. This is currently possible in Git
for Windows by setting the <Command> tag equal to

  C:\Program Files\Git\git-bash.exe

with options "--hide --no-needs-console --command=cmd\git.exe"
followed by the arguments currently used. Since git-bash.exe is not
included in Windows builds of core Git, I chose to leave out this
feature. My plan is to submit a small patch to Git for Windows that
converts the use of git.exe with this use of git-bash.exe in the
short term. In the long term, we can consider creating this GUI
shim application within core Git, perhaps in contrib/.

Co-authored-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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