Enable building of release candidate builds (#788)

The existing build-git-installers.yml workflow did not handle building
release candidate builds correctly, mainly because it hasn't been
exercised with a real release candidate build.

During the rc-stages of v2.51.0, various issues were uncovered:

- The tag validation was looking for the wrong format. Release candidate
tags look like `v2.X.Y-rcN.vfs.W.Z`, but it was checking for a tag like
`v2.X.Y.vfs.W.Z.rcN`.
- The GIT-VERSION-GEN script does some munging of the tag to get to the
_build_ version. Specifically the `-rc` part is replaced with `.rc` and
the leading `v` is dropped.. so a tag of `v2.51.0-rc0.vfs.0.0` will
become `2.51.0.rc0.vfs.0.0`. The macOS installed Makefile and scripts to
create the Debian package did not correctly account for this, leading to
build strings that were not consistent with those for Windows builds.

With these changes, we can now successfully build release candidate
versions. Tags must be of the form:

```text
v2.X.Y-rcN.vfs.W.Z
```

The resulting build version (output from `git --version`) looks like:

```text
2.X.Y.rcN.vfs.W.Z
```

Installer and package file names maintain the _tag_ version format, for
example:

- Git-2.51.0-rc0.vfs.0.0-{64-bit,arm64}.exe
- PortableGit-2.51.0-rc0.vfs.0.0-{64-bit,arm64}.7z.exe
- git-2.51.0-rc0.vfs.0.0-universal.{pkg,dmg}
- microsoft-git_2.51.0-rc0.vfs.0.0.deb


See an example successful build here:
https://github.com/microsoft/git/actions/runs/16830521090

VFS for Git recently learned to correctly parse these release candidate
version strings here: microsoft/VFSForGit#1854

Author: mjcheetham

.github/macos-installer/Makefile

@@ -21,7 +21,7 @@ GIT_PREFIX := $(PREFIX)/git
 # Replace -rc with .rc in the version string
 # This is to ensure compatibility with the format as generated by GIT-VERSION-GEN
 ORIGINAL_VERSION := $(VERSION)
-VERSION := $(shell echo $(ORIGINAL_VERSION) | sed 's/-rc/.rc/g')
+VERSION := $(subst -rc,.rc,$(VERSION))
 
 BUILD_DIR := $(GITHUB_WORKSPACE)/payload
 DESTDIR := $(PWD)/stage/git-$(ARCH_UNIV)-$(VERSION)
@@ -159,6 +159,6 @@ endif
 ifdef APPLE_KEYCHAIN_PROFILE
 notarize:
 	@$(CURDIR)/../scripts/notarize.sh \
-		--package="disk-image/git-$(VERSION)-$(ARCH_UNIV).pkg" \
+		--package="disk-image/git-$(ORIGINAL_VERSION)-$(ARCH_UNIV).pkg" \
 		--keychain-profile="$(APPLE_KEYCHAIN_PROFILE)"
 endif

.github/workflows/build-git-installers.yml

@@ -479,6 +479,7 @@ jobs:
 
           # Convert -rc to .rc to match GIT-VERSION-FILE behavior
           BUILD_VERSION=$(echo "${VERSION}" | sed 's/-rc/.rc/g')
+          echo "$BUILD_VERSION" >git/version
 
           # Configure universal build
           cat >git/config.mak <<EOF
@@ -601,6 +602,10 @@ jobs:
           }
 
           VERSION="${{ needs.prereqs.outputs.tag_version }}"
+
+          # Convert -rc to .rc to match GIT-VERSION-FILE behavior
+          BUILD_VERSION=$(echo "${VERSION}" | sed 's/-rc/.rc/g')
+          echo "$BUILD_VERSION" >git/version
           make -C git GIT-VERSION-FILE
 
           ARCH="$(dpkg-architecture -q DEB_HOST_ARCH)"