build: use a more user-friendly version string for shallow clones

If the cloned repo is shallow (e.g. like the default github actions
settings), the git describe command won't actually be able to fetch any
tags and will instead just get a hash. The resulting binary version
string will end up being that git hash. While it does tell you the exact
commit, it's not exactly helpful when wanting to know the general
version at a glance. For the case where we do have a git directory but
no available tags, build a version string using the mpv version + the
commit hash.

Fixes #15789.

common/meson.build

@@ -1,8 +1,27 @@
+git = find_program('git', required: false)
+if not git.found()
+    git = ''
+endif
+
+generate_ver = '''
+#!/usr/bin/env python3
+import os
+import sys
+from subprocess import DEVNULL, check_output
+git_cmd = [sys.argv[1], "--git-dir=" + os.path.join(sys.argv[2], ".git"),
+           "--work-tree=" + sys.argv[2]]
+describe_cmd = git_cmd + ["describe", "--abbrev=9", "--tags", "--dirty"]
+try:
+    ver = check_output(describe_cmd, stderr=DEVNULL, encoding="UTF-8")
+except Exception:
+    describe_cmd += ["--always"]
+    ver = check_output(describe_cmd, stderr=DEVNULL, encoding="UTF-8")
+    ver = f"v{sys.argv[3]}-dev-g{ver}"
+sys.stdout.write(ver)
+'''
+
 version_h = vcs_tag(
-    command: ['git',
-        '--git-dir=' + join_paths(source_root, '.git'),
-        '--work-tree=' + source_root,
-        'describe', '--always', '--tags', '--dirty'],
+    command: [python, '-c', generate_ver, git, source_root, meson.project_version().replace('-UNKNOWN', '')],
     fallback: 'v' + meson.project_version(),
     input: 'version.h.in',
     output: 'version.h',