Fix code coverage causing test failures

Summary:
Sandcastle's [1] code coverage builds compile EdenFS and Hg with Clang's -fprofile-generate flag (or a related flag). By default, running a program with with that flag creates a `default.profraw` file in the current directory. This causes some tests to fail, such as HgBackingStoreTest.getTreeForCommit_reimports_tree_if_it_was_deleted_after_import, because `default.profraw` is unintentionally committed to the test repo:

```
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from HgBackingStoreTest
[ RUN      ] HgBackingStoreTest.getTreeForCommit_reimports_tree_if_it_was_deleted_after_import
eden/fs/store/hg/test/HgBackingStoreTest.cpp:64: Failure
Value of: tree1->getEntryNames()
Expected: has 2 elements where
element #0 is equal to foo,
element #1 is equal to src
  Actual: { default.profraw, foo, src }, which has 3 elements
[  FAILED  ] HgBackingStoreTest.getTreeForCommit_reimports_tree_if_it_was_deleted_after_import (1563 ms)
[----------] 1 test from HgBackingStoreTest (1563 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (1563 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] HgBackingStoreTest.getTreeForCommit_reimports_tree_if_it_was_deleted_after_import
```

When Sandcastle runs the tests, it sets the `LLVM_PROFILE_FILE` environment variable, which configures the path to `default.profraw`. EdenFS' HgRepo class is stripping this variable when invoking hg, so hg uses the default path and not the configured path.

Make HgRepo pass `LLVM_PROFILE_FILE` through to hg. This fixes some of EdenFS' tests when run by Sandcastle for code coverage purposes. (This does *not* fix running the tests manually (without setting `LLVM_PROFILE_FILE`), though.)

[1] Sandcastle is Facebook's continuous integration system.

Reviewed By: simpkins

Differential Revision: D15104832

fbshipit-source-id: 3929b9b0ab0904f2552ace7d8e4e4f4a4221192c

eden/fs/testharness/HgRepo.cpp

@@ -63,7 +63,8 @@ HgRepo::HgRepo(AbsolutePathPiece path) : path_{path} {
   XLOG(DBG1) << "Using hg command: " << hgCmd_;
 
   // Set up hgEnv_
-  std::vector<const char*> passthroughVars{{"PATH", "HG_REAL_BIN"}};
+  std::vector<const char*> passthroughVars{
+      {"HG_REAL_BIN", "LLVM_PROFILE_FILE", "PATH"}};
   for (const char* varName : passthroughVars) {
     auto value = getenv(varName);
     if (value) {