build: add option to select a dependency branch.

We'll make use of this now, and as a general rule, it may be useful when
we need to easily point to feature branches for testing.

build/windows/1_build-deps-msys2.ps1

@@ -59,7 +59,7 @@ Invoke-Expression ((Get-Content $GIMP_DIR\.gitlab-ci.yml | Select-String 'win_en
 
 
 # Build babl and GEGL
-function self_build ([string]$dep, [string]$option1, [string]$option2)
+function self_build ([string]$dep, [string]$branch, [string]$option1, [string]$option2)
   {
     Write-Output "$([char]27)[0Ksection_start:$(Get-Date -UFormat %s -Millisecond 0):${dep}_build[collapsed=true]$([char]13)$([char]27)[0KBuilding $dep"
 
@@ -76,6 +76,10 @@ function self_build ([string]$dep, [string]$option1, [string]$option2)
             $git_options="--branch=$tag"
             Write-Output "Using tagged release of ${dep}: $tag"
           }
+        elseif ($branch)
+          {
+            $git_options="--branch=$branch"
+          }
         git clone $git_options --depth $GIT_DEPTH $repo
       }
     Set-Location $dep
@@ -98,7 +102,7 @@ function self_build ([string]$dep, [string]$option1, [string]$option2)
     Write-Output "$([char]27)[0Ksection_end:$(Get-Date -UFormat %s -Millisecond 0):${dep}_build$([char]13)$([char]27)[0K"
   }
 
-self_build babl
-self_build gegl
+self_build babl master
+self_build gegl master
 
 Set-Location $GIMP_DIR