Support choosing the date-format for the history pane's commit date - #693

First slice of #680. The date column's format was frozen in the nib
(dateStyle="long" timeStyle="short"), so every row read "19 September
2026 at 14:05" and nothing else was reachable.

• Add a Date format preference: the system short, medium and long
  styles, plus a free pattern for anyone who wants yyyy-MM-dd HH:mm.
  The default is the long style, so an existing install looks the same.
  The label column has no room to say which list the format belongs to,
  so the tooltip does: "Date format of History-list". It governs the
  Date column alone - the Relative Date column beside it is untouched.
• Format the column through PBGitCommitDateFormatter, which builds one
  formatter for every row on screen and rebuilds it only when the
  preference moves. The column formats once per visible row, so this
  costs what the nib's formatter cost.
• Mark the date cells that are already drawn for redraw when the format
  changes. A text field asks its formatter for a string as it draws,
  and the dates themselves do not change, so nothing marks them
  otherwise. Reloading the rows instead does not work here: the column
  is bound to arrangedObjects.date, so a reload re-hands the binding
  the commit it already had, the binding pushes nothing, and every row
  but the selected one keeps the format it was drawn with until it is
  scrolled away and back.
• Size the column from one reference date rendered in the format in
  force - 28 September 2026 at 22:58, a two digit day in a long month
  at a two digit hour - rather than from the dates on screen, so that a
  shorter format gives its room back and a longer one is not clipped the
  moment the list scrolls. It is an estimate the user can drag
  afterwards, and a date rather than a written down string, so it stays
  in the reader's own locale. The date is drawn 2pt in from each edge of
  its cell, so the width is ceil(text * 1.05) + 4, with the header as
  a floor since a pattern can render shorter than the word "Date".
• A pattern that is empty or all spaces falls back to the long style,
  and the rendered pattern shows under the field, so YYYY and friends
  are visible before they reach the list.
• Move the Git Executable row to Integration, at the top. It is the
  integration a git client rests on, and it frees the room in General
  that the date rows now take.

The relative-date column, NSRelativeDateTimeFormatter and the other
places a commit date is shown are the later slices of #680 and are not
touched here.

Test plan

• make unit-test - 166 tests pass, 18 of them new. Ten cover the
  formatter: each style against the system formatter of that name, a
  pattern used as written, an empty pattern falling back, a setting
  outside the enum falling back, a non-date value not formatted, the
  formatter in use following the preference, the change being
  announced, that the sizing date does not depend on today, that it is
  rendered by the format in force, and that it follows the preference.
  Eight cover the redraw and the resize, over a table built in the
  test: that the rows are drawn to begin with, that a format change
  asks every drawn date cell to redraw, that a list with no date column
  is left alone, that the column leaves room around the date, that a
  shorter format narrows it, that the header still fits, that a list
  with nothing to measure is left alone, and that a format change
  through the handler sizes the column. The redraw test was run against
  the previous implementation and fails there.
• ibtool --errors --warnings --notices on Preferences.xib against
  master: the only new notices are the intrinsic-height one every 17pt
  label in this nib already produces.
• In the running app: General is 500x334 and Integration 500x306, both
  keeping the 38pt top and 35pt bottom gutters. Picking Custom
  enables the pattern field and the sample becomes 2026-09-19 17:57.
• Read back out of the compiled nib: the pattern field is
  editable=YES selectable=YES as the guide-column fields beside it
  are, and the tooltip is on both the label and the popup. It was
  neither editable nor selectable before, so enabling it only
  recoloured its text - it could not be clicked into or tabbed to.