Clarify commands where we grep for a warning

In this justfile, for the `cargo tree ...| grep warning` commands:

- Put the explanatory comment before the first of these commands,
  rather than at the end of the first one, since this is easier to
  read in the `just` output, as well as clearer because it applies
  to a cluster of commands rather than only the first one. This is
  also more consistent stylistically with other comments in the
  `justfile`.

- Pass `-F` to `grep`, as this is a literal search for the text
  `warning`. (This is a minor refactoring for clarity, and it may
  be changed soon if the check is made more precise.)

Author: EliahKagan

justfile

@@ -50,10 +50,11 @@ check:
     ! cargo check -p gix-packetline --all-features 2>/dev/null
     ! cargo check -p gix-transport --all-features 2>/dev/null
     ! cargo check -p gix-protocol --all-features 2>/dev/null
-    cargo tree -p gix --no-default-features -e normal -i imara-diff 2>&1 | grep warning # warning happens if nothing found, no exit code :/
-    cargo tree -p gix --no-default-features -e normal -i gix-submodule 2>&1 | grep warning
-    cargo tree -p gix --no-default-features -e normal -i gix-pathspec 2>&1 | grep warning
-    cargo tree -p gix --no-default-features -e normal -i gix-filter 2>&1 | grep warning
+    # warning happens if nothing found, no exit code :/
+    cargo tree -p gix --no-default-features -e normal -i imara-diff 2>&1 | grep -F warning
+    cargo tree -p gix --no-default-features -e normal -i gix-submodule 2>&1 | grep -F warning
+    cargo tree -p gix --no-default-features -e normal -i gix-pathspec 2>&1 | grep -F warning
+    cargo tree -p gix --no-default-features -e normal -i gix-filter 2>&1 | grep -F warning
     ! cargo tree -p gix --no-default-features -i gix-credentials 2>/dev/null
     cargo check --no-default-features --features lean
     cargo check --no-default-features --features lean-async