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Git cacheing and better cloning in src/analysis/git.lisp

refs #1683

• src/analysis/git.lisp (clone-git-repository): new function; clone a
  git directory, potentially as a mirror
  (update-git-repository): new function; fetch remote date into an
  existing git repository
  (ensure-updated-git-repository): new function; clone or update a git
  repository
  (git-cache): new structure; thread-safe table mapping git urls to
  cache directories
  (git-cache): new variable; can be bound to the git cache
  (call-with-git-cache): new function; call a thunk with established git
  cache
  (with-git-cache): new macro; execute body with established git cache
  (clone-git-repository/cached): new function; clone/update cached
  mirror, then clone from cache
  (clone-git-repository/maybe-cached): new function; clone from cache or
  directly depending on whether a cache directory has been specified
  (analyze-git-branch): new function; helper for analyzing
  (clone-and-analyze-git-branch): new function; helper for cloning and
  analyzing
  (analyze uri eql :git): call `clone-and-analyze-git-branch'
• src/commandline-interface/main.lisp (analyze-project): accept
  cache-directory keyword parameter; pass to `analyze'
  (analyze-projects): accept cache-directory keyword parameter; pass to
  `analyze-project'; setup git cache
  (update-synopsis): added cache-directory option
  (main): pass cache directory to analyze-projects

Fixed caching in presence of sub-directories in src/analysis/git.lisp

refs #1683

• src/analysis/git.lisp (analyze-git-branch): renamed keyword parameter
  commit-key > key
  (analyze-git-branch/cached): likewise
  (analyze-git-branch/maybe-cached): likewise
  (clone-and-analyze-git-branch): use `%sub-directory>key' to
  incorporate sub-directory into cache key
  (%sub-directory->key): new function; incorporate sub-directory into
  cache key
• jenkins.project.asd (system jenkins.project): added system dependency
  on ironclad