core/transaction: first drop unmergable jobs for anchor jobs

As you can see, something spurious happens in the logs below.
```
initrd-switch-root.target: Trying to enqueue job initrd-switch-root.target/start/isolate
systemd-repart.service: Looking at job systemd-repart.service/stop conflicted_by=no
systemd-repart.service: Looking at job systemd-repart.service/start conflicted_by=no
systemd-repart.service: Fixing conflicting jobs systemd-repart.service/stop,systemd-repart.service/start by deleting job systemd-repart.service/stop
initrd-switch-root.target: Fixing conflicting jobs initrd-switch-root.target/stop,initrd-switch-root.target/start by deleting job initrd-switch-root.target/stop
systemd-repart.service: Deleting job systemd-repart.service/start as dependency of job initrd-switch-root.target/stop
```

The two conflicting jobs for systemd-repart.service are initially queued
as the following:
- initrd-switch-root.target has Wants=initrd-root-fs.target, and
  initrd-root-fs.target has Wants=systemd-repart.service (through symlink),
  hence starting initrd-switch-root.target tries to start
  systemd-repart.service,
- systemd-repart.service has Conflicts=initrd-switch-root.target, hence
  starting initrd-switch-root.target tries to stop
  systemd-repart.service.

As similar, interestingly(?) starting initrd-switch-root.target tries to
stop initrd-switch-root.target.

So, now there are at least two pairs of conflicting jobs:
- systemd-repart.service: start vs stop,
- initrd-switch-root.target: start vs stop.

As these jobs are induced by starting initrd-switch-root.target, of course
the most important one is the start job for initrd-switch-root.target.

Previously, as you can see in the logs at the beginning, even if
the start job for initrd-switch-root.target is important, we may first
try to resolve the conflict in systemd-repart.service, and may drop the
stop job for systemd-repart.service even if it is relevant to the start
job of initrd-switch-root.target.

This makes first we solve the pair of conflicting jobs for anchor task.
So the stop job for initrd-switch-root.target is dropped first, and the
induced start job for systemd-repart.service is automatically removed,
thus it is not necessary to solve the conflict in systemd-repart.service
anymore.

This is especially important for services that are enabled both in initrd
and after switching root. If a stop job for one of the service is
unexpectedly dropped during switching root, then the service is not stopped
before switching root, and will never start after that.

Fixes #38765.

src/core/transaction.c

@@ -15,6 +15,7 @@
 #include "strv.h"
 #include "transaction.h"
 
+static bool job_matters_to_anchor(Job *job);
 static void transaction_unlink_job(Transaction *tr, Job *j, bool delete_dependencies);
 
 static void transaction_delete_job(Transaction *tr, Job *j, bool delete_dependencies) {
@@ -216,17 +217,18 @@ static int delete_one_unmergeable_job(Transaction *tr, Job *job) {
         return -EINVAL;
 }
 
-static int transaction_merge_jobs(Transaction *tr, sd_bus_error *e) {
+static int transaction_ensure_mergeable(Transaction *tr, bool matters_to_anchor, sd_bus_error *e) {
         Job *j;
         int r;
 
         assert(tr);
 
-        /* First step, check whether any of the jobs for one specific
-         * task conflict. If so, try to drop one of them. */
         HASHMAP_FOREACH(j, tr->jobs) {
                 JobType t;
 
+                if (job_matters_to_anchor(j) != matters_to_anchor)
+                        continue;
+
                 t = j->type;
                 LIST_FOREACH(transaction, k, j->transaction_next) {
                         if (job_type_merge_and_collapse(&t, k->type, j->unit) >= 0)
@@ -253,7 +255,26 @@ static int transaction_merge_jobs(Transaction *tr, sd_bus_error *e) {
                 }
         }
 
-        /* Second step, merge the jobs. */
+        return 0;
+}
+
+static int transaction_merge_jobs(Transaction *tr, sd_bus_error *e) {
+        Job *j;
+        int r;
+
+        assert(tr);
+
+        /* First step, try to drop unmergeable jobs for jobs that matter to anchor. */
+        r = transaction_ensure_mergeable(tr, /* matters_to_anchor = */ true, e);
+        if (r < 0)
+                return r;
+
+        /* Second step, do the same for jobs that not matter to anchor. */
+        r = transaction_ensure_mergeable(tr, /* matters_to_anchor = */ false, e);
+        if (r < 0)
+                return r;
+
+        /* Third step, merge the jobs. */
         HASHMAP_FOREACH(j, tr->jobs) {
                 JobType t = j->type;