string repeat: Don't allocate repeated string all at once (#9124)

* string repeat: Don't allocate repeated string all at once

This used to allocate one string and fill it with the necessary
repetitions, which could be a very very large string.

Now, it instead uses one buffer and fills it to a chunk size,
and then writes that.

This fixes:

1. We no longer crash with too large max/count values. Before they
caused a bad_alloc because we tried to fill all RAM.
2. We no longer fill all RAM if given a big-but-not-too-big value. You
could've caused fish to eat *most* of your RAM here.
3. It can start writing almost immediately, instead of waiting
potentially minutes to start.

Performance is about the same to slightly faster overall.

tests/checks/string.fish

@@ -504,6 +504,32 @@ string repeat -n3 ""
 or echo string repeat empty string failed
 # CHECK: string repeat empty string failed
 
+# See that we hit the expected length
+# First with "max", i.e. maximum number of characters
+string repeat -m 5000 aab | string length
+# CHECK: 5000
+string repeat -m 5000 ab | string length
+# CHECK: 5000
+string repeat -m 5000 a | string length
+# CHECK: 5000
+string repeat -m 17 aab | string length
+# CHECK: 17
+string repeat -m 17 ab | string length
+# CHECK: 17
+string repeat -m 17 a | string length
+# CHECK: 17
+# Then with "count", i.e. number of repetitions.
+# (these are count * length long)
+string repeat -n 17 aab | string length
+# CHECK: 51
+string repeat -n 17 ab | string length
+# CHECK: 34
+string repeat -n 17 a | string length
+# CHECK: 17
+# And a more tricksy case with a long string that we truncate.
+string repeat -m 5 (string repeat -n 500000 aaaaaaaaaaaaaaaaaa) | string length
+# CHECK: 5
+
 # Test equivalent matches with/without the --entire, --regex, and --invert flags.
 string match -e x abc dxf xyz jkx x z
 or echo exit 1