Add path mtime (#9057)

This can be used to print the modification time, like `stat` with some
options.

The reason is that `stat` has caused us a number of portability
headaches:

1. It's not available everywhere by default
2. The versions are quite different

For instance, with GNU stat it's `stat -c '%Y'`, with macOS it's `stat
-f %m`.

So now checking a cache file can be done just with builtins.

tests/checks/path.fish

@@ -198,3 +198,21 @@ test (path resolve link) = (pwd -P)/link
 and echo link resolves to link
 # CHECK: link resolves to link
 
+
+# path mtime
+# These tests deal with *time*, so we have to account
+# for slow systems (like CI).
+# So we should only test with a lot of slack.
+
+echo bananana >> foo
+test (math abs (date +%s) - (path mtime foo)) -lt 20
+or echo MTIME IS BOGUS
+
+sleep 2
+
+set -l mtime (path mtime --relative foo)
+test $mtime -ge 1
+or echo mtime is too small
+
+test $mtime -lt 20
+or echo mtime is too large