setup.py: handle git not being installed

My previous change to call git unconditionally caused Packit failures,
presumably because git isn't installed for the build:

Traceback (most recent call last):
  File "/builddir/build/BUILD/drgn-0.0.22+71.g2f97cc0/setup.py", line 411, in <module>
    version=get_version(),
            ^^^^^^^^^^^^^
  File "/builddir/build/BUILD/drgn-0.0.22+71.g2f97cc0/setup.py", line 343, in get_version
    subprocess.call(
  File "/usr/lib64/python3.11/subprocess.py", line 389, in call
    with Popen(*popenargs, **kwargs) as p:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/subprocess.py", line 1024, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib64/python3.11/subprocess.py", line 1901, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'git'

We shouldn't bother calling git if .git doesn't exist.

Fixes: 2e9e66c ("setup.py: use stricter check for git repository")
Signed-off-by: Omar Sandoval <[email protected]>

Author: osandov

setup.py

@@ -339,7 +339,7 @@ def get_version():
 
     # If this is a git repository, use a git-describe(1)-esque local version.
     # Otherwise, get the local version saved in the sdist.
-    if (
+    if os.path.exists(".git") and (
         subprocess.call(
             ["git", "--git-dir=.git", "rev-parse"], stderr=subprocess.DEVNULL
         )