schema: Support more checkout data in v4.5

Add support for three more checkout fields: `git_commit_tags`,
`git_commit_message`, and `git_repository_branch_tip`.

The `git_commit_tags` is an array of strings representing annotated tags
pointing directly at the commit being checked out, as seen in the
source repository. I.e. the output of `git tag --points-at <commit>`.
Set to an empty array, if the commit has no tags.

The `git_commit_message` is intended to hold the complete message of the
commit being checked out, *both* subject and body. I.e. the output of
`git show -s --format=%B`. We're putting the subject and the body
together, as it's quite easy to extract the subject in SQL, while
full-text search is easier and more efficient to do over a single
column.

Finally, the `git_repository_branch_tip` is a boolean flag, which should
be set to `true`, when the commit being checked out is at the tip of the
branch at the moment of the checkout (as specified in `start_time`).
Essentially, if you're always testing only the tip of the branch, you
can set this to `true` unconditionally. This flag would let us extract
the checkouts which represented the branch state over time, and produce
a rough history of branch changes, which we can then use for
(regression) analysis and graphs, in lieu of actual commit graph
walking.

kcidb_io/schema/v04_05.py

@@ -237,12 +237,46 @@ class Version(PreviousVersion):
                             "the checked out base source code, as would be "
                             "output by \"git describe\", at the checkout time."
                     },
+                    "git_commit_tags": {
+                        "type": "array",
+                        "description":
+                            "The list of (annotated) tags, found in the "
+                            "checked-out repository, pointing directly "
+                            "at the commit being checked out. I.e. as "
+                            "output by \"git tag --points-at <commit>\".",
+                        "items": {
+                            "type": "string",
+                            "description":
+                                "A git tag pointing at the checked-out commit"
+                        },
+                    },
+                    "git_commit_message": {
+                        "type": "string",
+                        "description":
+                            "The complete message of the commit being "
+                            "checked-out, both the subject and the body. "
+                            "I.e. as output by \"git show -s --format=%B\".",
+                    },
                     "git_repository_branch": {
                         "type": "string",
                         "description":
                             "The Git repository branch from which the commit "
                             "with the base source code was checked out."
                     },
+                    "git_repository_branch_tip": {
+                        "type": "boolean",
+                        "description":
+                            "True if at the moment of checkout (specified in "
+                            "\"start_time\") the checked out commit was at "
+                            "the tip of the specified branch in the "
+                            "specified repository. False if it was further "
+                            "back in history.\n"
+                            "\n"
+                            "This information is used to reconstruct the "
+                            "approximate history of the branch changes for "
+                            "display and analyzis, in lieu of actual commit "
+                            "graph walking."
+                    },
                     "patchset_files": {
                         "description":
                             "List of patch files representing the patchset "