Always Update Git information when rebuilding (#4696)

Commit 61734b41b7b96e7e7cbf46021a357d840d64b42e changed the way some of
our source files are generated. However, the change meant that once
`git_versioninfo.cpp` was generated, it was never updated again: The
custom command for `git_versioninfo.cpp` has no dependencies, so CMake
does not rebuild it unless the output file is missing [0]. This commit
reverts the change to our `gen-gitinfo` target and adds `git_versioninfo.cpp`
to `BYPRODUCTS` for the target to indicate that the file may have changed.
I am not sure if there is a better solution because we actually have to run
`GitInfo.cmake` to see if there have been any changes in the Git information. 
Introducing a dependency on all source files is not sufficient because other 
files or just the branch name may change. Note: The original solution only
updates the timestamp of `git_versioninfo.cpp` if its contents actually
change (`GitInfo.cmake` uses `configure_file()` to generate
`git_versioninfo.cpp`, which only updates the timestamp when the
contents changed [1]), so we don't do any unnecessary work.

[0] https://cmake.org/cmake/help/latest/command/add_custom_command.html
[1] https://cmake.org/cmake/help/latest/command/configure_file.html

Signed-off-by: Andrew V. Jones <andrew.jones@vector.com>
Co-authored-by: Andrew V. Jones <andrew.jones@vector.com>

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