tests/fetchGit: work around something that looks a bash parsing quirk.

Before we got something like this but only inside the VM test:

 vm-test-run-functional-tests-on-nixos_user> machine # fetchGit.sh: line 286: unexpected EOF while looking for matching `)'

We now try to do not too much in a single line, so that the bash parser does not get confused.
This also seems more readable and better quoted.

Author: Mic92

tests/functional/fetchGit.sh

@@ -280,17 +280,20 @@ path11=$(nix eval --impure --raw --expr "(builtins.fetchGit ./.).outPath")
 empty="$TEST_ROOT/empty"
 git init "$empty"
 
-emptyAttrs='{ lastModified = 0; lastModifiedDate = "19700101000000"; narHash = "sha256-pQpattmS9VmO3ZIQUFn66az8GSmB4IvYhTTCFn6SUmo="; rev = "0000000000000000000000000000000000000000"; revCount = 0; shortRev = "0000000"; submodules = false; }'
-
-[[ $(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]") = $emptyAttrs ]]
+emptyAttrs="{ lastModified = 0; lastModifiedDate = \"19700101000000\"; narHash = \"sha256-pQpattmS9VmO3ZIQUFn66az8GSmB4IvYhTTCFn6SUmo=\"; rev = \"0000000000000000000000000000000000000000\"; revCount = 0; shortRev = \"0000000\"; submodules = false; }"
+result=$(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]")
+[[ "$result" = "$emptyAttrs" ]]
 
 echo foo > "$empty/x"
 
-[[ $(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]") = $emptyAttrs ]]
+result=$(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]")
+[[ "$result" = "$emptyAttrs" ]]
 
 git -C "$empty" add x
 
-[[ $(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]") = '{ lastModified = 0; lastModifiedDate = "19700101000000"; narHash = "sha256-wzlAGjxKxpaWdqVhlq55q5Gxo4Bf860+kLeEa/v02As="; rev = "0000000000000000000000000000000000000000"; revCount = 0; shortRev = "0000000"; submodules = false; }' ]]
+expected_attrs="{ lastModified = 0; lastModifiedDate = \"19700101000000\"; narHash = \"sha256-wzlAGjxKxpaWdqVhlq55q5Gxo4Bf860+kLeEa/v02As=\"; rev = \"0000000000000000000000000000000000000000\"; revCount = 0; shortRev = \"0000000\"; submodules = false; }"
+result=$(nix eval --impure --expr "builtins.removeAttrs (builtins.fetchGit $empty) [\"outPath\"]")
+[[ "$result" = "$expected_attrs" ]]
 
 # Test a repo with an empty commit.
 git -C "$empty" rm -f x