CMake: use git rev-parse to get GIT_COMMIT_HASH

The old approach of reading .git/HEAD does not work when using git
worktrees, where the folder layout looks roughly like:

solvespace.git/                      - bare clone (.git dir)
solvespace.git/work                  - example worktree containing master
solvespage.git/worktrees/work/       - .git dir of worktree
solvespage.git/worktrees/work/HEAD   - actual HEAD ref for master

First attempt was to just get GIT_ROOT from `git rev-parse --git-dir` but
that wasn't enough, since:

1. GIT_ROOT points to solvespage.git/worktrees/work/
2. GIT_ROOT/HEAD points to refs/heads/master
3. GIT_ROOT/refs/heads/master does not exist but the old implementation
   would want to use this to get the sha

so we need two invocations of git rev-parse

1. `git rev-parse --git-dir` to get GIT_DIR
    needed for setting GIT_DEPENDS
2. `git rev-parse HEAD` to get the sha of the worktree's HEAD

cmake/GetGitCommitHash.cmake

@@ -1,31 +1,11 @@
 function(get_git_commit_hash)
-    get_filename_component(GIT_DESCRIBE_CMAKE_DIR ${CMAKE_CURRENT_LIST_FILE} PATH)
-    get_filename_component(GIT_ROOT ${GIT_DESCRIBE_CMAKE_DIR} PATH)
-    set(GIT_DIR "${GIT_ROOT}/.git")
+    execute_process(COMMAND git rev-parse --git-dir OUTPUT_VARIABLE GIT_DIR OUTPUT_STRIP_TRAILING_WHITESPACE)
+    execute_process(COMMAND git rev-parse HEAD OUTPUT_VARIABLE HEAD_REF OUTPUT_STRIP_TRAILING_WHITESPACE)
 
     # Add a CMake configure dependency to the currently checked out revision.
     set(GIT_DEPENDS ${GIT_DIR}/HEAD)
-    file(READ ${GIT_DIR}/HEAD HEAD_REF)
-    if(HEAD_REF MATCHES "ref: (.+)\n")
-        set(HEAD_REF ${CMAKE_MATCH_1})
-        if(EXISTS "${GIT_DIR}/${HEAD_REF}")
-            list(APPEND GIT_DEPENDS ${GIT_DIR}/${HEAD_REF})
-            file(READ ${GIT_DIR}/${HEAD_REF} HEAD_REF)
-        elseif(EXISTS "${GIT_DIR}/packed-refs")
-            list(APPEND GIT_DEPENDS ${GIT_DIR}/packed-refs)
-            file(READ "${GIT_DIR}/packed-refs" PACKED_REFS)
-            if(${PACKED_REFS} MATCHES "([0-9a-z]*) ${HEAD_REF}")
-                set(HEAD_REF ${CMAKE_MATCH_1})
-            else()
-                set(HEAD_REF "")
-            endif()
-        else()
-            set(HEAD_REF "")
-        endif()
-    endif()
     set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${GIT_DEPENDS})
 
-    string(STRIP ${HEAD_REF} HEAD_REF)
     if(HEAD_REF STREQUAL "")
         message(WARNING "Cannot determine git HEAD")
     else()