pkgs/build-support/fetchgit: add fetchTags parameter

Added fetchTags feature to fetchgit, explicit and clear support for
fetching all tags after the source tree fetch completes. Doing this at
build-time in the fetcher is required for packages that invoke commands
like 'git describe' which require tags, and since the nix store is
read-only by design, it is not possible to git fetch --tags at
activation- or run-time. This feature may have been possible by
specifying a postFetch option including calling git fetch --tags,
however doing so obfuscates the solution to this very real problem.
Explicit support for fetching tags should be a first class citizen just
like fetching other refs.

doc/build-helpers/fetchers.chapter.md

@@ -795,6 +795,10 @@ Additionally, the following optional arguments can be given:
 : Clone the entire repository as opposing to just creating a shallow clone.
   This implies `leaveDotGit`.
 
+*`fetchTags`* (Boolean)
+
+: Whether to fetch all tags from the remote repository. This is useful when the build process needs to run `git describe` or other commands that require tag information to be available. This parameter implies `leaveDotGit`, as tags are stored in the `.git` directory.
+
 *`sparseCheckout`* (List of String)
 
 : Prevent git from fetching unnecessary blobs from server.