populate: refer to flakes as git+file: only with a usable .git

Writing the lock file of a git+file: flake, Nix runs Git, with the
repository's configuration and hooks, which may run commands.  A .git
in a source's directory can come from anywhere, e.g. one planted in a
materialized flake, or one a file source keeps.  So drawing inputs
refers to a flake as git+file: only if git_problems finds nothing wrong
with its .git directory, the way git sources check their work trees,
and else as path:.

lock-foreign-git plants a .git in a materialized flake and in a file
source that keeps it, whose configuration names an fsmonitor command.

1 files changed, 4 insertions, 1 deletions

diff --git a/README.md b/README.md
index e46ef4b..94568f6 100644
--- a/README.md
+++ b/README.md
@@ -724,7 +724,10 @@ drawing an input from a source with a generated flake uses that flake.
 automatically.
 
 Sources in a Git work tree are referred to as `git+file:` (with submodules if
-there are any), all other sources as `path:`, with symlinks resolved.  Lock
+there are any), all other sources as `path:`, with symlinks resolved.  Nix
+runs Git in a `git+file:` flake, so that's only for work trees whose `.git`
+directory a `git` source would use, see
+[The target directory](#the-target-directory).  Lock
 files get rewritten or generated on every run, so they match the sources.
 Copies of `flake` and `flake-input` sources get made again only if the
 delivered store path, the patches, or the sources they draw inputs from