Minor updates to inline comments

Add missing docstring on InstallableCommand. Also, some of these were wrapped
when they're right next to a line longer than the unwrapped line, so we can just
unwrap them to save vertical space.

src/libexpr/primops/fetchGit.cc

@@ -45,9 +45,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
         if (!clean) {
 
-            /* This is an unclean working tree. So copy all tracked
-               files. */
-
+            /* This is an unclean working tree. So copy all tracked files. */
             GitInfo gitInfo;
             gitInfo.rev = "0000000000000000000000000000000000000000";
             gitInfo.shortRev = std::string(gitInfo.rev, 0, 7);