Have git verify the repository

[theory]

Rather than simply check for the presence of .git when git-save exists, run git rev-parse --is-inside-work-tree and replace the .git directory when it returns an error. This will fix corrupted .git directories left over from previous builds, as well as missing .git directories.

Was inspired to make this change when my build farm animal started failing builds after a couple of runs with this error:

baza:REL_17_STABLE [14:38:24] checking out source ...
Missing checked out branch bf_REL_17_STABLE:
fatal: not a git repository (or any of the parent directories): .git
Branch: REL_17_STABLE
Stage pgsql-Git failed with status 1

The .git directory did in fact exist, but there wasn't much in it:

$ ls -la build/REL_17_STABLE/pgsql/.git/
total 16
drwxr-xr-x 4 animal animal 4096 Aug 10 23:18 .
drwxr-xr-x 3 animal animal 4096 Aug 10 23:20 ..
drwxr-xr-x 2 animal animal 4096 Aug 10 23:18 info
drwxr-xr-x 3 animal animal 4096 Aug 10 23:18 objects

But git-save had much more:

ll build/REL_17_STABLE/git-save/
total 752K
-rw-r--r-- 1 animal animal 4.1K Aug 10 23:18 FETCH_HEAD
-rw-r--r-- 1 animal animal   33 Aug 10 20:50 HEAD
-rw-r--r-- 1 animal animal   41 Aug 10 23:18 ORIG_HEAD
lrwxrwxrwx 1 animal animal   41 Aug 10 17:28 config -> /home/animal/build/HEAD/pgsql/.git/config
-rw-r--r-- 1 animal animal   96 Aug 10 23:18 gc.log.lock
-rw-r--r-- 1 animal animal   14 Aug 10 23:18 gc.pid
lrwxrwxrwx 1 animal animal   40 Aug 10 17:28 hooks -> /home/animal/build/HEAD/pgsql/.git/hooks
-rw-r--r-- 1 animal animal 724K Aug 10 23:18 index
lrwxrwxrwx 1 animal animal   39 Aug 10 17:28 info -> /home/animal/build/HEAD/pgsql/.git/info
drwxr-xr-x 2 animal animal 4.0K Aug 10 23:18 logs
lrwxrwxrwx 1 animal animal   42 Aug 10 17:28 objects -> /home/animal/build/HEAD/pgsql/.git/objects

I have no idea how or why the directory was corrupted, but this patch fixes the issue by restoring an uncorrupted .git directory.