MissingObjectException in repos with git alternates #178
Description
It seems JGit's RevWalk doesn't support Git alternates. This leads to sbt-git being unable to load projects checked out by TeamCity's VCS root that has "Use mirrors" mechanism enabled. For example,
$ pwd
/data/buildagent/work/aee92f3311be76c5
$ ls .git/objects/a8/04e5fbec3088c2890667aa46712c96b02b5dea
ls: cannot access .git/objects/a8/04e5fbec3088c2890667aa46712c96b02b5dea: No such file or directory
$ cat .git/objects/info/alternates
/data/homedir/buildAgent/system/git/git-0C318551.git/objects
$ ls /data/homedir/buildAgent/system/git/git-0C318551.git/objects/a8/04e5fbec3088c2890667aa46712c96b02b5dea
/data/homedir/buildAgent/system/git/git-0C318551.git/objects/a8/04e5fbec3088c2890667aa46712c96b02b5dea
org.eclipse.jgit.errors.MissingObjectException: Missing unknown a804e5fbec3088c2890667aa46712c96b02b5dea
at org.eclipse.jgit.internal.storage.file.WindowCursor.open(WindowCursor.java:163)
at org.eclipse.jgit.lib.ObjectReader.open(ObjectReader.java:234)
at org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:859)
at org.eclipse.jgit.revwalk.RevWalk.parseCommit(RevWalk.java:772)
at com.typesafe.sbt.git.JGit.$anonfun$headCommitDate$1(JGit.scala:115)
at scala.Option.map(Option.scala:230)
at com.typesafe.sbt.git.JGit.headCommitDate(JGit.scala:114)
at com.typesafe.sbt.SbtGit$.$anonfun$buildSettings$8(SbtGit.scala:117)
at com.typesafe.sbt.git.DefaultReadableGit.withGit(ReadableGit.scala:42)
at com.typesafe.sbt.SbtGit$.$anonfun$buildSettings$7(SbtGit.scala:117)
at scala.Function1.$anonfun$compose$1(Function1.scala:49)
at sbt.internal.util.EvaluateSettings$MixedNode.evaluate0(INode.scala:228)
at sbt.internal.util.EvaluateSettings$INode.evaluate(INode.scala:170)
at sbt.internal.util.EvaluateSettings.$anonfun$submitEvaluate$1(INode.scala:87)
at sbt.internal.util.EvaluateSettings.sbt$internal$util$EvaluateSettings$$run0(INode.scala:99)
at sbt.internal.util.EvaluateSettings$$anon$3.run(INode.scala:94)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)