Fix 'git fetch --prune' deleting all remote refs.

Before Git 1.7.7.2 'git fetch --prune origin master' used to remove all remote references.
This was fixed in 1.7.7.2.
For the sake of simplicity we are not going to call '--prune' for the older Gits.

Other way would be to call 'git remote prune origin' for older Gits,
but this auto-prune feature doesn't seem to be important enough for such effort.

plugins/git4idea/src/git4idea/config/GitVersionSpecialty.java

@@ -73,10 +73,15 @@ public enum GitVersionSpecialty {
     }
   },
 
+  /**
+   * {@code git fetch --prune} is actually supported since 1.7.0,
+   * but before 1.7.7.2 calling {@code git fetch --prune origin master} would delete all other references.
+   * This was fixed in {@code ed43de6ec35dfd4c4bd33ae9b5f2ebe38282209f} and added to the Git 1.7.7.2 release.
+   */
   SUPPORTS_FETCH_PRUNE {
     @Override
     public boolean existsIn(@NotNull GitVersion version) {
-      return version.isLaterOrEqual(new GitVersion(1, 7, 0, 0));
+      return version.isLaterOrEqual(new GitVersion(1, 7, 7, 2));
     }
   },