[GHC] #16253: Offer a shorthand for `--skip=_build/stage$n/compiler/.dependencies.mk`

[GHC]

#16253: Offer a shorthand for `--skip=_build/stage$n/compiler/.dependencies.mk`
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           Reporter:  sgraf          |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Build System   |           Version:  8.6.3
  (Hadrian)                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When working on a stage1 compiler, the slightest change in any of the
 files leads to rebuilding the dependency matrix, which takes 20-30s time.
 That makes for a very disruptive edit-compile cycle.

 Alp helped me on #ghc and found
 `--skip=_build/stage0/compiler/.dependencies.mk` as the right flag to skip
 dependency rebuilding. I wonder if could hide that behind a nicer flag? I
 think this should do similar things as `--freeze1`, only that we 'freeze'
 stage 0 and dependency building.

 The analogy is that we need a hadrian equivalent of `make -C ghc 1` as we
 have `--freeze1` for `make -C ghc 2`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16253>
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